Modulation of Cell Death in Yeast by the Bcl-2 Family of Proteins

Abstract

Bcl-2 family members are regulators of cell death. The precise biochemical properties of these proteins are unclear although intrafamily protein-protein association is thought to be involved. To elucidate structure-activity relationships among Bcl-2 proteins and identify the pathways in which they act, an inducible death suppressor assay was developed in yeast. Only Bax and Bak killed yeast via a process that did not require interleukin-1beta-converting enzyme-like proteases. Bax/Bak lethality was suppressed by coexpression of Bcl-2 family members that are anti-apoptotic in vertebrates, namely Bcl-xL, Bcl-2, Mcl-1, and A1. Furthermore, Bcl-xL and Bcl-2 suppressed Bax toxicity by distinct mechanisms in yeast. Bad, Bcl-xS, and Ced-9 lacked suppressor activity. These inactive proteins bound to anti-apoptotic members of the Bcl-2 family but not to Bax or Bak. In contrast, most Bcl-2 family proteins that attenuated death bound to Bax and Bak. However, two mutants of Bcl-xL suppressed Bax-induced cell death while having no Bax binding activity. Therefore, Bcl-xL functions independently of Bax binding, perhaps by interacting with a common target or promoting a pathway that antagonizes Bax. Thus, the pathways downstream of Bax and Bcl-xL may be conserved between vertebrates and yeast. This suppressor assay could be used to isolate components of these pathways.

Full text

Modulation of Cell Death in Yeast by the Bcl-2 Family of Proteins*
(Received for publication, February 27, 1997)
Weikang Tao‡§, Cornelia Kurschner‡, and James I. Morgan‡¶
From the ‡Department of Developmental Neurobiology, St. Jude Children’s Research Hospital, Memphis, Tennessee 38105
and the §Department of Biochemistry and Molecular Biology, University of Medicine and Dentistry of New Jersey-
Graduate School of Biochemical Sciences, Newark, New Jersey 07103
Bcl-2 family members are regulators of cell death. The
precise biochemical properties of these proteins are un-
clear although intrafamily protein-protein association
is thought to be involved. To elucidate structure-activity
relationships among Bcl-2 proteins and identify the
pathways in which they act, an inducible death suppres-
sor assay was developed in yeast. Only Bax and Bak
killed yeast via a process that did not require interleu-
kin-1
b
-converting enzyme-like proteases. Bax/Bak le-
thality was suppressed by coexpression of Bcl-2 family
members that are anti-apoptotic in vertebrates, namely
Bcl-xL, Bcl-2, Mcl-1, and A1. Furthermore, Bcl-xL and
Bcl-2 suppressed Bax toxicity by distinct mechanisms in
yeast. Bad, Bcl-xS, and Ced-9 lacked suppressor activity.
These inactive proteins bound to anti-apoptotic mem-
bers of the Bcl-2 family but not to Bax or Bak. In con-
trast, most Bcl-2 family proteins that attenuated death
bound to Bax and Bak. However, two mutants of Bcl-xL
suppressed Bax-induced cell death while having no Bax
binding activity. Therefore, Bcl-xL functions independ-
ently of Bax binding, perhaps by interacting with a com-
mon target or promoting a pathway that antagonizes
Bax. Thus, the pathways downstream of Bax and Bcl-xL
may be conserved between vertebrates and yeast. This
suppressor assay could be used to isolate components of
these pathways.
Cell death is a highly regulated process involving interac-
tions among extracellular molecules, intracellular signal trans-
duction pathways, and resident suicide/rescue programs (1, 2).
Studies in Caenorhabditis elegans have pointed to a cell suicide
pathway that includes several molecules that have homologs in
vertebrates (3). Central among these death-regulating proteins
is Ced-9, which suppresses programmed cell death in C. el-
egans (4). Bcl-2, the vertebrate homolog of Ced-9, was identified
independently through its translocation in many B cell follicu-
lar lymphomas in man (5, 6). Bcl-2 inhibits cell death in various
circumstances in vertebrate cells and functionally substitutes
for Ced-9 in C. elegans (7, 8). Subsequent investigations have
identified a number of proteins in vertebrates that are struc-
turally related to Bcl-2 (7). These proteins constitute a family,
the members of which share a number of regions of homology,
termed BH1 (Bcl-2 homology), BH2, and BH3 domains (9, 10).
Some of these Bcl-2 related proteins, such as Bcl-xL, also pre-
vent cell death, whereas others, such as Bax, provoke cell
elimination. The biochemical and biophysical mechanisms that
confer these properties on the Bcl-2 family of proteins remain
enigmatic, although recent structural data suggest that they
may be pore-forming proteins (11). However, Bcl-2 and many of
its related proteins can participate in homo- and heteromeric
complexes, and it has been suggested that the activity of pro-
apoptotic members of the Bcl-2 family is neutralized by their
association with anti-apoptotic members (12, 13).
There are a number of caveats in the interpretation of the
role of Bcl-2 family members in the regulation of cell death in
vertebrates. First, the effects of Bcl-2 family members are often
assessed in models where cell death is triggered by an exoge-
nous means such as growth factor withdrawal or addition of a
toxin or virus to the culture medium. While this is more rele-
vant to the physiological situation, it adds a level of ambiguity
as to whether the effects are mediated through intrafamily
interactions. Second, a given cell type may already express the
gene of interest as well as other known and potentially un-
known members of the Bcl-2 family. Thus, there is uncertainty
as to precisely which proteins interact to produce the observed
effect. Finally, Bcl-2 family members are differentially ex-
pressed, and bcl-2- and bcl-x-null mice have distinct pheno-
types (14, 15). Together, these data imply that the various
Bcl-2-like proteins have functional differences. Indeed, there
are indications that Bcl-xL need not dimerize with Bax to
suppress cell killing and that Bcl-2 and Bcl-xL have differential
activities in some assays (14–18). Thus, from the mechanistic
standpoint, there is a need for a model in which the role of Bcl-2
family members in cell death can be determined without the
foregoing ambiguities.
Recently, several studies reported that the expression of Bax
is lethal in the budding yeast, Saccharomyces cerevisiae (19–
21). This is despite the fact that yeasts express no identifiable
members of the Bcl-2 family and are not known to undergo
programmed cell death. This afforded the opportunity to de-
velop a suppressor assay in which the ability of Bcl-2 family
members to attenuate Bax killing could be determined quanti-
tatively. Moreover, since the model is in essence the same as a
two-hybrid system, protein-protein association can be assessed
simultaneously.
It is shown that Bcl-2, Bcl-xL, Mcl-1, and A1 can suppress
death induced by Bax and Bak in yeast, whereas Ced-9, Bad,
and Bcl-xS are inactive. Bcl-2 and Bcl-xL do not have identical
structure-activity relationships for death suppression in yeast,
indicating that they have distinct modes of action. Moreover,
several Bcl-xL mutants suppressed Bax lethality without bind-
ing to Bax, indicating that Bcl-xL can function by a mechanism
other than direct association with Bax. This implies that mech-
anisms downstream of Bcl-xL and Bax are conserved in yeast
and that this system could be used to isolate these molecules.
* This work was supported in part by National Institutes of Health
Cancer Support CORE Grant P30 CA21765 and by the American Leb-
anese Syrian Associated Charities. The costs of publication of this
article were defrayed in part by the payment of page charges. This
article must therefore be hereby marked “advertisement” in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
¶To whom correspondence should be addressed: Dept. of Develop-
mental Neurobiology, St. Jude Children’s Research Hospital, 332 N.
Lauderdale, Memphis, TN 38105. Tel.: 901-495-2256; Fax: 901-495-
3143; E-mail: jim.morgan@stjude.org.
THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 272, No. 24, Issue of June 13, pp. 15547–15552, 1997
© 1997 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.
This paper is available on line at http://www.jbc.org 15547
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EXPERIMENTAL PROCEDURES
Yeast Strains, Growth, and Transformation—The S. cerevisiae strain
S260 (ura3 trp1) contained a genomic LEXA-operator-LACZ fusion
reporter gene and was described previously (22). Yeast growth, main-
tenance, and transformations were as described (23).
Yeast Expression Constructs—Fusion proteins with the LexA DNA
binding domain were constructed in the yeast expression plasmid,
Y.LexA (22), which carries the S. cerevisiae TRP1 gene as a selectable
marker. Fusion proteins with the VP16 transcriptional activation do-
main were generated in pSD.10a, which harbors the URA3 selection
marker (24). Constructs lacking heterologous fusion sequences were
made in pSD.10a after deletion of the VP16 codons.
LexA and VP16 fusions of murine Bax, Bcl-2, and A1 were reported
previously (22). cDNAs encoding murine Bcl-x, Bak, and Bad were
obtained by reverse transcription polymerase chain reaction using
mouse brain RNA and polymerase chain reaction primers based upon
published sequences. Murine mcl-1 cDNA was isolated in a yeast two-
hybrid screening for Bax-binding proteins.
1
ACed-9 cDNA was a gift
from Dr. M. O. Hengartner, and plasmid p996 containing a crmA cDNA
(25) was a gift from Dr. D. Pickup. Truncation and deletion mutants
were generated by polymerase chain reaction. The sequences of all
constructs were verified.
In Vitro Translation Constructs—For in vitro transcription/transla-
tion reactions, cDNAs encoding full-length Bcl-xL and two of its mu-
tants (XF14 and XF15) were inserted into the vector pT7
b
plink (24). A
full-length murine bax cDNA was cloned in pT7
b
plink-TagN (22). In
this construct, an epitope tag derived from human c-MYC protein is
fused to the N terminus of Bax. This epitope is recognized by the
monoclonal antibody 9E10.
Coimmunoprecipitation—Proteins were translated in vitro as de-
scribed (22). MYC-tagged Bax was translated in the absence of [
35
S]me-
thionine (Amersham Life Science, Inc.), and untagged Bcl-xL and its
mutants XF14 and XF15 were translated in the presence of [
35
S]methi-
onine. For coimmunoprecipitations, 2
m
l of the Bax-MYC translation
reaction were mixed with 10
m
l of the Bcl-xL, XF14, or XF15 translation
reactions, respectively. Incubation and washing steps were performed
as described (26) in NETgel buffer with 0.2% Nonidet P-40. The precip-
itating antibody was 9E10 (Santa Cruz Biotechnology, Inc.). Protein
A-Sepharose CL-4B (Sigma) was used to precipitate the immune
complexes.
Yeast Two-hybrid Analysis—S260 was transformed with two plas-
mids encoding a LexA fusion construct and a VP16 hybrid. Transform-
ants were grown and assayed for
b
-galactosidase activity as described
(22). The development of blue color in the yeast colonies was monitored
for 24 h.
Yeast Growth Assay—S260 was (co)transformed with expression
plasmids encoding Bcl-2 family members. Selective media containing
2% glucose was inoculated with a single colony of transformants and
incubated overnight at 30 °C. Subsequently, cells were washed three
times with H
2
O. Typically, 20 ml of selective medium (with 2% galac-
tose) was inoculated with 2.56 310
7
cells, and incubation was contin-
ued. Samples were taken at different time points, and cell density was
measured by determining the OD at 660 nm. To compare results
from different experiments, a growth index (GI)
2
was devised.
(OD
660/22 h
2OD
660/0 h
(cells containing Bax or Bak and the test
protein))/(OD660/22 h 2OD
660/0 h
(cells containing Bax or Bak and LexA or
VP16)).
Immunoblotting—Yeast cells were lysed mechanically as described
(26). Proteins were separated on 15% SDS-polyacrylamide gels (30
m
g/lane) and transferred to nitrocellulose membranes. Immunostaining
was performed in Tris-buffered saline containing 1% fetal calf serum at
room temperature. Membranes were incubated with a rabbit anti-
murine Bax polyclonal antibody, 13686E (1:1000 dilution) (Pharmin-
gen), followed by horseradish peroxidase-conjugated goat anti-rabbit
immunoglobulin antiserum (1:1000) (Amersham), both for 2 h. Immu-
noblots were developed using diaminobenzidine as chromogen (27). For
Bcl-xL immunostaining, the membrane was stripped of bound antibody,
reprobed with a monoclonal anti-Bcl-x antibody, B22620 (1:250) (Trans-
duction Laboratories). The immunoblots were developed using the en-
hanced chemiluminescence method (Amersham).
RESULTS
Identification of Members of the Bcl-2 Family That Are Le-
thal in Yeast—Since Bax kills yeast (19–21), additional mem-
bers of the Bcl-2 family (Bcl-2, Bcl-xL, Bcl-xS, A1, Mcl-1, Bad,
Bak, and Ced-9) were tested for this property. Besides Bax,
only Bak was lethal, although it was consistently less potent
(Table I). Moreover, like native Bax, the LexA and VP16 fusion
proteins of Bax and Bak used in the two-hybrid assay were also
lethal (Fig. 1, Aand B, and Table I), making it possible to
correlate binding activity with biological activity in subsequent
studies.
Suppression of Bax- and Bak-induced Lethality by Bcl-2
Family Members—To quantitatively determine the suppres-
sion of Bax/Bak toxicity in yeast, cDNAs encoding LexA or
VP16 fusions of various Bcl-2 family members were coex-
pressed with either Bax or Bak. Coexpression of the Bcl-xL
fusion proteins generated for the two-hybrid system inhibited
Bax and Bak toxicity (Fig. 1Aand Table I). In addition, LexA
fusions of Bcl-2, Mcl-1, and A1 inhibited both Bax and Bak
killing (Fig. 2 and Table I). Bcl-xS, Bad, and Ced-9 did not
attenuate Bax or Bak lethality (Table I). To ensure that the
results were not artifacts of LexA or VP16 fusion proteins,
these data were confirmed using native (unfused) protein se-
quences. Both native Bcl-2 (GI, 7.4) and native Bcl-xL (GI, 8.0)
suppressed the killing elicited by native Bax (GI, 1.0).
Relationship between the Anti-death Activity of Bcl-2 Family
Members and Their Binding to Bax and Bak—In the yeast
two-hybrid assay, Bcl-xL, Bcl-2, Mcl-1, and A1 all bound to Bax
and Bak, whereas Bcl-xS, Bad, and Ced-9 did not (Table I).
Therefore, anti-death activity was associated with the ability to
bind to Bax or Bak. Proteins such as Ced-9, Bcl-xS, and Bad
that did not bind to Bax/Bak did not inhibit killing, whereas
proteins that could associate, such as Bcl-xL and Mcl-1, were
inhibitory. Since the lack of death suppressor activity of Ced-9
was unexpected, its association with anti-apoptotic members of
the Bcl-2 family was determined. LexA-Ced-9 (as well as LexA-
Bcl-xS and LexA-Bad) bound to Bcl-xL, Bcl-2, and A1 (data not
shown). Therefore, Ced-9 had the binding and activity profiles
of proteins such as Bcl-xS rather than Bcl-2.
Mutations in Bcl-xL Dissociate Bax Binding from Death Sup-
pressor Activity—To further pursue the relationship between
heterodimerization and biological activity, a series of trunca-
tion and deletion mutations of Bcl-2 and Bcl-xL were made.
1
C. Kurschner and J. I. Morgan, unpublished data.
2The abbreviations used are: GI, growth index; ICE, interleukin-1
b
-
converting enzyme; TM, transmembrane.
TABLE I
Relationship between heterodimerization and suppression of cell death for Bcl-2 family members in yeast
S260 yeast were cotransformed with either VP16-fused Bax or Bak and one of the indicated Bcl-2 family members fused to LexA. Association
between the proteins was determined by the two-hybrid assay. 1, blue colony within 1 h; 2, no blue colony after 24 h. The death-suppressive
activities of the Bcl-2 family members were determined in the yeast growth assay. GI values are indicated. ND, not done; NA, not applicable. The
data shown are representative of three independent experiments.
Y.LexA Bcl-2 Bcl-xL Bcl-xS A1 Mcl-1 Bad Ced-9 Bax Bak
Bax
Binding 2 1 1 2 1 1 2211
Suppression 1.00 5.18 5.55 1.01 3.60 8.05 0.82 1.10 NA ND
Bak
Binding 2 1 1 2 1 1 2211
Suppression 1.00 29.15 29.87 0.97 27.75 28.44 1.24 1.12 0.13 NA
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When coexpressed with Bax, the majority of these mutants
failed to bind to Bax and did not suppress death (Fig. 2, Aand
B). However, two Bcl-xL mutants, XF14 and XF15, were active
in suppression of Bax toxicity but either bound weakly or not
all in the two-hybrid assay (Fig. 2A).
To ensure that these data were not a product of the LexA
sequences, unfused XF14 and XF15 were examined for their
suppressive effects on native Bax toxicity and for coimmuno-
precipitation with MYC-tagged Bax. Both unfused XF14 (GI,
8.7) and XF15 (GI, 8.8) were potent suppressors of Bax lethality
(GI, 1.0). For coimmunoprecipitation, the proteins were in vitro
translated in the presence of [
35
S]methionine, whereas a MYC
epitope-tagged Bax (MYC-Bax) was translated in the absence
of the radionuclide. After appropriate mixing and incubation,
Bcl-xL was specifically immunoprecipitated with MYC-Bax but
not the MYC epitope alone (Fig. 3). In contrast, neither XF14
nor XF15 was precipitated specifically with MYC-Bax and the
anti-MYC monoclonal antibody (Fig. 3). XF15 showed no evi-
dence of Bax binding. XF14 gave a relatively high background
immunoprecipitation that was not augmented by the presence
of MYC-Bax, indicating that it does not bind to Bax. This
analysis confirms a prior report using these and similar con-
structs (28).
Bcl-xL and Bcl-xL Mutants Do Not Alter Bax Expression—To
preclude the possibility that coexpression of Bcl-xL or the mu-
tants either reduced the expression or promoted the degrada-
tion of Bax, steady-state protein levels were determined by
FIG.2. Structure-activity relationships for Bcl-xL and Bcl-2
binding to Bax and suppression of Bax lethality in yeast. Panel A,
yeasts were transformed with Bax and an additional plasmid encoding
LexA, LexA-Bcl-xL, or a LexA fusion of a mutated Bcl-xL. Growth was
assessed after 22 h by measuring cell density at OD
660
and is expressed
as Growth Index (see “Experimental Procedures” for details). The bind-
ing of the LexA fusion proteins to VP16-Bax was determined using the
two-hybrid assay. Colonies that turned blue within 1 h were scored as
positive (1), whereas those that were still white at 24 h were considered
negative (2). Three mutants were negative at 1 h but weakly positive at
3 h when present as LexA fusions but were negative at 24 h when
present as VP16 fusions. These clones were scored as having marginal
Bax binding activity (6). The structures of the various Bcl-xL mutants
are shown at the left. The major Bcl-2 homology (BH) domains as well
as the transmembrane region (TM) are indicated by shaded boxes. The
numbers over the constructs refer to the respective amino acids in the
full-length Bcl-xL sequence that define particular mutants. In one
mutant, XF15, amino acids 26–82 were deleted and replaced with the
sequence AAAAVAAAA (amino acid single letter code) (A
4
VA
4
). The
data shown are representative of three independent experiments. Panel
B, the same series of VP16-Bax binding and suppression experiments
were performed for Bcl-2 and a number of its mutants. Details of the
growth and two-hybrid assays are as for panel A, as are domain no-
menclature and amino acid numbering. In one Bcl-2 mutant, BF6,
residues 32–87 were deleted and replaced with four alanines (A
4
).
FIG.1. Bcl-xL suppresses Bax-induced lethality in yeast. A,
yeasts were transformed with plasmids encoding LexA, LexA-Bax,
LexA-Bax and Bcl-xL, or LexA-Bax and VP16. Cell density was deter-
mined at various time intervals by measuring OD
660
.B, yeasts were
transformed with plasmids encoding VP16, Bax, Bax and LexA, or Bax
and LexA-Bcl-xL. Growth was assessed at various times by measuring
OD
660
. The data are representative curves that were repeated a mini-
mum of three times.
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immunoblotting. Fig. 4Ashows the immunoblot analysis of Bax
and Bcl-xL proteins in yeast. Cells were cotransformed with
constructs encoding LexA-Bax and one of unfused Bcl-xL mu-
tants, XF14 or XF15. An ;45-kDa LexA-Bax fusion protein
accumulated to maximal levels within 12 h after galactose
induction. Expression of ;26-kDa Bcl-xL, ;20-kDa XF14, and
;21-kDa XF15 immunoreactive bands were also detected 12 h
after galactose induction. None of the anti-death proteins al-
tered the levels or time course of LexA-Bax expression (Fig.
4A). Additional Bax-reactive proteins of lower molecular mass
were also induced with similar kinetics to LexA-Bax. The levels
of these proteins were also unaffected by Bcl-xL expression
(Fig. 4A). It is unclear whether these bands are proteolytic
fragments of LexA-Bax or incomplete transcription/translation
products. No such proteins were observed when unfused Bax
was expressed with Bcl-xL in yeast (Fig. 4B), suggesting that
the additional bands do not represent selective processing of
Bax sequences.
Whereas expression of Bax is lethal, yeast transformed with
the Bax plasmid did grow if cultured for longer periods (data
not shown). This phenomenon is important in that it has im-
plications for the use of this system as a suppressor screening
assay. The growth effect could arise in a number of ways. First,
a mutation in the vector could either inhibit expression of Bax
or render it biologically inactive. Second, yeast may produce
their own suppressor of Bax or lose a target of Bax. To test
these possibilities, yeasts were cotransformed with Bax and
Bcl-xL and subjected to immunoblot analysis after 100 h of
culture. Fig. 4Bshows that cells containing Bax and LexA
plasmids no longer expressed immunoreactive Bax. However,
an ;21-kDa Bax was still present in the cells containing Bax
and LexA-Bcl-xL. Thus, mutations are selected for in Bax-
expressing yeast that result in a loss of Bax expression. In the
presence of Bcl-xL, this selective pressure is absent, and high
(normally lethal) levels of full-length Bax are expressed. This
effect must be considered when using this suppressor assay to
identify proteins that functionally interact with Bax/Bak.
Bax Toxicity May Not Be Mediated by ICE-like Proteases—
Since Bax and Bcl-xL may function independently, the mech-
anisms that mediate Bax toxicity were investigated. Another
study has established that Bax lethality in yeast is not associ-
ated with DNA laddering (21), although it may involve prote-
olysis. Indeed, proteases belonging to the interleukin-1
b
-con-
verting enzyme (ICE) subfamily (recently termed caspases)
have been implicated in cell death in phylogenetically dispar-
ate species (2). A search of the yeast genome for the consensus
active site of ICE proteases (QACRG) yielded no hits, suggest-
ing that these enzymes could not mediate death in yeast. To
further examine this point, yeasts were cotransformed with
Bax and CrmA, an inhibitor of ICE proteases that is derived
from cowpox virus (25). CrmA did not rescue Bax toxicity and
alone had no effect upon yeast growth (data not shown). To-
gether, the data indicate that ICE-like proteases do not medi-
ate Bax lethality in yeast.
Bcl-2 and Bcl-xL Have Distinct Structure-Activity Relation-
ships in Yeast—Several studies have suggested that Bcl-2 and
Bcl-xL have distinct properties (15, 17, 18). Therefore, the
structure-activity relationships for Bax dimerization and sup-
pression were determined for the two molecules in yeast. Two
regions of Bcl-2 and Bcl-xL distinguished the biological prop-
erties of the two proteins. We confirm that the transmembrane
(TM) domain of Bcl-2 is not essential for suppression of Bax
toxicity in yeast and its elimination does not affect Bax binding
(see mutant BF3 in Fig. 2B) (21). However, elimination of the
TM domain in Bcl-xL leads to both the loss of Bax binding and
suppressor activity (see mutant XF3 in Fig. 2A). As shown
above, deletion of the putative loop region in Bcl-xL (mutants
FIG.3. Coimmunoprecipitation of Bcl-xL and Bcl-xL mutants
with MYC-tagged Bax. Either unlabeled MYC-tagged Bax (MYC-Bax)
or the MYC epitope (MYC) were incubated with [
35
S]methionine-la-
beled Bcl-xL, XF14, or XF15 (see “Experimental Procedures” for de-
tails). Subsequently, immunoprecipitation was carried out using the
monoclonal anti-MYC antibody 9E10. Bcl-xL showed a low but detect-
able background precipitation with the MYC epitope alone. However,
Bcl-xL showed markedly enhanced immunoprecipitation in the pres-
ence of MYC-Bax, indicating specific association between Bcl-xL and
Bax. Mutant XF15 was not precipitated by either the MYC epitope or
MYC-Bax, indicating a lack of association of this mutant with Bax.
Mutant XF14 gave a relatively high background with MYC epitope
alone, but unlike Bcl-xL, this signal was not augmented by MYC-Bax.
M
r
(K), molecular mass in kilodaltons.
FIG.4. Evaluation of the levels of Bcl-2 family members in
yeast by immunoblotting. A, yeasts were cotransformed with LexA-
Bax and a plasmid encoding VP16, Bcl-xL, XF14, or XF15. Transform-
ants were switched to galactose medium, and the levels of the various
proteins were determined by sequential immunoblotting as described
under “Experimental Procedures.” The position of full-length LexA-Bax
is indicated. Several inducible lower molecular mass species are also
evident. The upper panel shows the immunoblot of LexA-Bax, whereas
the lower panel depicts the same blot reprobed for Bcl-xL or its two
mutants. The positions of Bcl-xL, XF14, and XF15 are indicated at the
right, and molecular mass is at the left. Coexpression of Bcl-xL or the
two mutants did not affect the level or processing of any of the Bax-
reactive bands. B, yeasts were cotransformed with VP16 and LexA, Bax
and LexA-Bcl-xL, or Bax and LexA. Transformants were switched to
galactose medium for 100 h, and extracts were immunoblotted for Bax
as described under “Experimental Procedures.” Unlike LexA-Bax, na-
tive Bax gave a single band at the appropriate molecular mass. Bax was
only detectable in cultures that coexpressed Bax with LexA-Bcl-xL.
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XF14 and XF15) (11) results in a loss of Bax binding but
retention of Bax suppressor activity (Fig. 2A). Whereas an
equivalent mutation in Bcl-2 (BF6) retained suppressor activ-
ity, it still bound well to Bax (Fig. 2B). These data suggest that
the loop region may be important for the interaction of Bcl-xL
with Bax, whereas the equivalent domain in Bcl-2 is not. To-
gether, the results indicate that whereas Bcl-2 and Bcl-xL,
both, can suppress Bax toxicity in yeast, they may not do so in
an identical manner.
DISCUSSION
As in vertebrate cell death models, members of the Bcl-2
family can be grouped into three functional classes in yeast.
The first group comprises proteins, such as Bax and Bak, which
are lethal per se. The second group includes Bcl-2, Bcl-xL,
Mcl-1, and A1, which bind to and suppress Bax and Bak lethal-
ity. The third group bind to anti- but not pro-apoptotic mem-
bers of the Bcl-2 family. These proteins, which include Bad,
Bcl-xS, and the C. elegans Ced-9, are functionally inactive in
terms of direct killing or death suppression. In vertebrates,
proteins such as Bad and Bcl-xS are not considered to be lethal
per se but rather are thought to potentiate killing by binding to
anti-apoptotic members of the Bcl-2 family (29, 30). Thus,
mammalian Bcl-2 family members have the same spectrum of
biological activities in yeast as they do in vertebrate cells.
One unexpected result was that whereas Ced-9 did bind to
anti-apoptotic members of the Bcl-2 family, it did not bind to
Bax and did not suppress Bax/Bak killing. Although we can
find no study that has used Ced-9 to rescue death in a verte-
brate cell, it is the presumed homolog of Bcl-2 (8) and was
expected to suppress killing. However, the properties of Ced-9
are more akin to those of Bcl-xS than they are to Bcl-2. It is
conceivable that Ced-9 binds specifically to a C. elegans hom-
olog of Bax, although no such gene has been identified. How-
ever, the possibility exists that family members such as Ced-9
and Bcl-xS might exert their functions through mechanisms
other than intrafamily binding.
Genetic analysis in C. elegans has shown that Ced-9 acts via
Ced-4, a protein of unknown function, and Ced-3, an ICE-like
cysteine protease (4). It is possible that Ced-9, and by implica-
tion Bcl-xS and Bad, might act by binding to Ced-4 or related
proteins in vertebrates. However, whereas many studies have
implicated cysteine proteases in cell death in vertebrates (2), it
is unlikely that ICE-like proteases mediate Bcl-2 family effects
in yeast. First, Bax lethality is not inhibited by coexpression of
the ICE protease inhibitor, CrmA. Second, no consensus se-
quence for the active site of ICE proteases has been found in
the yeast genome. Therefore, Bax lethality may not involve the
activation of ICE-like proteases in yeast. Indeed, Bax killing
has been shown to be independent of this class of proteases in
at least one vertebrate model (31).
Analysis of the structural requirements of Bcl-xL for sup-
pression of Bax toxicity in yeast revealed that deletions at both
the N and C termini eliminated biological activity. In one
mutant, XF3, deletion of the last 22 amino acids, which in-
cluded the TM domain, resulted in loss of suppressor activity.
This result suggests that membrane targeting is essential for
the protective effect of Bcl-xL in yeast. However, the equivalent
domain in Bcl-2 is not required for rescue (19), indicating that
these two related proteins may have divergent mechanisms of
action. Such a notion is underscored by the distinct phenotypes
of mice that lack functional bcl-2 or bcl-x alleles and by the
unique protective effects of Bcl-xL in some cell death models
(14, 15, 17, 18). Whereas the TM domain is thought to be
necessary for the activity of Bcl-xL in vertebrate cells, the
situation for Bcl-2 is controversial (12, 32, 33). However, the
recent demonstration that Bcl-xL has structural similarities to
the pore-forming subunit of diphtheria toxin suggests that its
biological function may occur at or within cellular membranes
(11). Therefore it is conceivable that there are proteins in yeast
that can dock Bcl-2, but not Bcl-xL, to membranes thereby
obviating the requirement for a TM domain. Bcl-2 and Bcl-xL
can also be discriminated by their structure-activity relation-
ship for binding to Bax. Mutants of Bcl-xL that had the puta-
tive loop domain deleted (XF14 and XF15) did not bind Bax,
whereas an equivalent deletion in Bcl-2 (BF6) did bind in the
two-hybrid assay. Since the loop regions of Bcl-2 and Bcl-xL are
not conserved, these data may be an indication that these
domains can selectively modify dimerization, although they
may not be part of the dimerization interface.
Dimerization between Bcl-2 family members is considered
central to their biological activity. Indeed, in one model it is the
stoichiometry of various pro- to anti-apoptotic Bcl-2 family
members that is supposed to determine cell fate (34). However,
the view that dimerization is the sole determinant of activity
has been questioned. For example, some Bcl-xL mutants that
failed to associate with Bax still rescued 70–80% of Sindbis
virus-induced cell death (16). However, it could be argued that
Sindbis virus-triggered cell death may not be mediated by Bax.
Indeed, there is evidence of Bax-independent cell death path-
ways in bax-null mice (35). Recently, some Bcl-xL mutations
were described that had reduced, or no, binding activity to Bax
but that rescued vertebrate cells from death triggered by IL-3
deprivation (11, 28). Therefore we compared the activity of
these deletions with other Bcl-xL and Bcl-2 mutants as sup-
pressors of Bax toxicity in yeast. All full-length Bcl-2 family
members and mutants that bound to Bax inhibited killing,
whereas those that did not bind were inactive. However, the
two internal deletion mutants of Bcl-xL, XF14 and XF15, that
did not bind to Bax were potent suppressors. This indicates
that dimerization is not essential for Bcl-xL to suppress Bax
lethality in yeast.
The observation that Bcl-xL can antagonize Bax killing in
yeast independent of heterodimerization has several important
implications. First, it argues against the effects of Bax in yeast
as being nonspecific toxicity that can be attenuated by any
Bax-binding protein. Second, it suggests that Bax and Bcl-xL
either interact with, or compete for, a common downstream
target or pathway in yeast. Alternatively, they could influence
antagonistic mechanisms. Third, the data suggest that Bax and
Bcl-xL act upon mechanisms that have been conserved from
yeast to mammals, although these processes may only have
been adapted for the control of cell elimination in multicellular
organisms. The assay described here can be used as a genetic
suppressor screen to identify potential components of this path-
way in yeast and vertebrates.
Acknowledgments—We thank Dr. Craig B. Thompson for helpful
discussions and Dr. Steven Dalton for providing the yeast expression
plasmids pSD.10a and Y.LexA, in vitro translation plasmids, and the S.
cerevisiae reporter strain S260.
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Weikang Tao, Cornelia Kurschner and James I. Morgan
Modulation of Cell Death in Yeast by the Bcl-2 Family of Proteins
doi: 10.1074/jbc.272.24.15547
1997, 272:15547-15552.J. Biol. Chem.
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