Nuclear mutations conferring oligomycin resistance in Neurospora crassa

Abstract

Mutants of Neurospora crassa have been isolated that are highly resistant to inhibition by oligomycin, an inhibitor of mitochondrial ATPase activity. Dixon plots (Dixon, M., and Webb, E.C. (1964) Enzymes, 2nd Ed, pp. 328-330, Academic Press, New York) of oligomycin inhibition curves of the parent strain and the resistant mutants are linear, indicating that oligomycin interacts at a single site within the ATPase complex. The Ki values obtained from the mutants vary from 150 to 900 times greater than the Ki obtained for the parent strain. The parent strain and the oligomycin-resistant mutants are also inhibited by bathophenanthroline, a lipophilic chelating agent that inhibits F1 ATPase activity. Dixon plots of bathophenanthroline inhibition curves are also linear and Ki values obtained are all approximately equal. Crosses of the oligomycin-resistant mutants to the oligomycin-sensitive parent strain show a mendelian segregation of the resistance characteristic. These data show that mutations leading to oligomycin resistance in Neurospora are due to alterations in nuclear genes.

Full text

Nuclear Mutations Conferring Oligomycin Resistance in Neurospora
crassa*
(Received for publication, December 22, 1977)
David L. Edward@ and Barbara W. Unger
From the Department of Biochemistry, Scripps Clinic and Research Foundation, La Jolla, California 92037
Mutants ofNeurospora crassa have been isolated that
are highly resistant to inhibition by oligomycin, an
inhibitor of mitochondrial ATPase activity. Dixon plots
(Dixon, M., and Webb, E. C. (1964) Enzyqws, 2nd Ed, pp.
328-330, Academic Press, New York) of oligomycin
inhibition curves of the parent strain and the resistant
mutants are linear, indicating that oligomycin interacts
at a single site within the ATPase complex. The K,
values obtained from the mutants vary from 150 to 900
times greater than the K, obtained for the parent strain.
The parent strain and the oligomycin-resistant mutants
are also inhibited by bathophenanthroline, a lipophilic
chelating agent that inhibits F1 ATPase activity. Dixon
plots of bathophenanthroline inhibition curves are also
linear and K, values obtained are all approximately
equal. Crosses of the oligomycin-resistant mutants to
the oligomycin-sensitive parent strain show a mende-
lian segregation of the resistance characteristic. These
data show that mutations leading to oligomycin resist-
ance in Neurospora are due to alterations in nuclear
genes.
A useful approach in the study of mitochondrial biogenesis
and of mitochondrial bioenergetics has been to utilize simple
eukaryotic organisms to select for mutants that are resistant
to inhibitors of these processes. Such mutants can be mapped
genetically and characterized biochemically, yielding insights
into the mechanism of action of the processes being studied.
This approach has been utilized most successfully in the yeast
Saccharomyces cereuisiae
where many drug-resistant strains
have been isolated. Many of these mutations segregate in an
extrachromosomal manner and map to distinct loci on mito-
chondrial DNA (cfi Fig. 2 of Ref. 1).
The fungus
Neurospora crassa
has also been utilized in
studies of mitochondria1 biogenesis. In this organism, however,
it has not been possible to select for mutants that are resistant
to inhibitors of mitochondrial function or assembly. This is
due in part to the fact that wild type strains of
Neurospora
have the capacity to produce alternate mitochondrial respi-
ratory pathways that can be induced and utilized when the
standard mitochondrial electron transfer system malfunctions
either due to mutation or to inhibition (2, 3). The net result
is that the alternate respiratory pathways allow the organism
to survive under conditions that would otherwise be lethal
(i.e.
in the presence of an inhibitor of mitochondrial electron
* This work was supported by Grant GM-24991 from the National
Institute of General Medical Sciences (D. L. E.). The costs of publi-
cation of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked “aduertise-
ment” in accordance with 18 USC. Section 1734 solely to indicate
this fact.
f Recipient of a Research Career Development Award from the
National Institute of General Medical Sciences.
transport such as antimycin A or of mitochondrial oxidative
phosphorylation such as oligomycin). Therefore, it has not
been possible to select for mutants that are resistant to such
drugs because wild type cells grow even in the presence of
high concentrations of them. For example, antimycin A-re-
sistant respiration has been described in mutants of
Neuro-
spora
by several investigators (4,5). The resistant phenotype,
however, is due to the presence of an alternative respiratory
pathway that is induced in these cells and bypasses the
antimycin A block (5, 6).
We have recently described (7, 8) two alternate respiratory
pathways that are present in a standard strain of
Neurospora
(i&-89601) used in our laboratory. One pathway is inhibited
by substituted hydroxamic acids and has a high respiratory
capacity (-100 ~1 of OJh/mg). The other is inhibited by azide
in the presence of saturating amounts of both cyanide and
hydroxamic acid and has a low respiratory capacity (-5 ~1 of
O,/h/mg). The presence of either of these pathways is suffi-
cient to allow the organism to survive in the presence of high
concentrations of drugs such as antimycin A (8). We have also
described a mutant, ANT-l, that is deficient in both the
hydroxamate-sensitive and azide-sensitive pathways (7, 9).
This mutant can respire only by the mitochondrial cyto-
chrome chain and cannot produce the hydroxamate-sensitive
or azide-sensitive pathway under any conditions.
In this communication, we report the use of ANT-l to select
for mutants that are resistant to oligomycin, an inhibitor of
mitochondrial ATPase activity. The mutants that we describe
are all highly resistant to the drug and all are due to nuclear
mutations.
MATERIALS AND ME?‘HODS
Strains-The ANT-l strain used in these studies has been de-
scribed previously (9).
Nomenclature-All of the oligomycin-resistant isolates have been
given the prefix oli followed by an experiment number and an isolate
number. Thus, mutant oli 16-7 is the seventh oligomycin-resistant
isolate selected from Experiment 16. This designation is meant only
to differentiate between oligomycin-resistant isolates and does not
imply
genetic loci.
Selection
of
Oligomycin-resistant Mutants-Conidia from ANT-l
were mutagenized with ultraviolet light and plated directly on plates
containing Vogel& medium N (10) supplemented with inositol (50 pg/
ml), sorbose (l%), fructose (0.05%), glucose (0.05%), and containing
oligomycin (5 pg/ml). Conidia from ANT-I did not grow when plated
directly onto this medium. Colonies that grew on the oligomycin
plates were isolated and further tested on growth tubes containing
Vogels’ medium N, supplemented with inositol (5 pg/ml), sucrose
(2%), and oligomycin (5 pg/ml). Growth tube experiments were ac-
cording to Rosenberg et al. (11). Isolates that showed vigorous growth
on growth tubes were retained for direct measurements of mitochon-
drial ATPase activity.
Preparation of Submitochondrial Particles-Conidia from ANT-
1 or the oligomycin-resistant isolates were inoculated at a concentra-
tion
of 5 x 10”/ml in Vogels’ medium N supplemented as above
without oligomycin and grown for 12 h at 30°C on a Gyrotory shaker.
Mycelium was harvested by filtration on a Buchner funnel. The cells
4254
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Oligomycin Resistance in Neurospora 4255
were suspended in mitochondrial preparation medium (6) and were
broken by grinding with alumina
in
an ice cold mortar and pestle.
Cell debris was removed by filtration through cheesecloth. The re-
sultant slurry was centrifuged twice at 1,000 X g for 5 min to remove
cell debris and alumina. Mitochondria were isolated by centrifuging
the supernatant solution at 9,000 X g for 15 min. Submitochondrial
particles were prepared from the mitochondrial pellet according to
Mainzer and Slayman (12). The mitochondrial pellet was resuspended
in 10 mM Tris/SO,, pH 7.5, to give a final protein concentration of 10
to 20 mg/ml. The suspension was then sonicated for 1% min using a
Branson 200 cell disruptor (macro tip, 40% duty cycle, output setting
= 8). The sonicated solution was immediately added to a chilled
centrifuge tube containing the same Tris/SO, buffer and a volume of
glycerol sufficient to give a final glycerol concentration of IO%.’ The
suspension was centrifuged at 120,000 x g for 45 min. The pellet was
resuspended in 10 mM Tris/SO,, pH 7.5, 10% glycerol to give a final
concentration of 5 to 10 mg of protein/ml. Particles prepared in this
manner had high levels of ATPase activity and could be stored at
-75°C without appreciable loss of activity.
ATPase Assays-ATPase activity was measured using a modifi-
cation (13) of the method of Pullman et al. (14). In a l-ml quartz
cuvette were added 25 pmol of Tris/acetate (pH 7.5), 25 pmol of
potassium acetate, 300 pmol of sucrose, 2 pmol of MgC12, 0.4 pmol of
NADH, 1 pmol of phosphoenolpyruvate, 2.8 units of lactic dehydro-
genase, and 3.0 units of pyruvate kinase to a final volume of 0.98 ml.
The mixture was equilibrated at 3O’C. Submitochondrial particles (50
to 80 pg) were added and incubated with varying amounts of either
oligomycin or bathophenanthroline. The incubation was carried out
for 5 min with oligomycin and 2 min with bathophenanthroline.
Bathophenanthroline was added as the tridentate chelate of ferrous
iron. The reaction was started by the addition of 2 pmol of ATP (pH
7). The decrease of NADH, which is a measure of equimolar amounts
of ADP formed from ATP, was followed at 340 nm (E~~+,~ = 6.22 mM ’
cm ‘). A plot of the change in absorbance at 340 nm as a function of
the concentration of submitochondrial particles was found to be linear
for protein concentrations as high as 160 pg. All of the assays that we
report here were well within that range. Protein was determined by
the method of Lowry et al. (15) using bovine serum albumin as a
standard.
Genetic Studies-Crosses were carried out on slants of corn meal
agar as described by Davis and De Serres (16). Shot spores were
collected in sterile water and germinated by heat shock at 65’C for 45
min. Oligomycin resistance was scored in tubes containing 1 ml of
Vogels’ medium supplemented as above and containing 5 pg of oli-
gomycin. The tubes were scored visually for growth after 3 days at
30°C.
Chemicals-Pyruvate kinase was obtained from Calbiochem. Ru-
tamycin was a generous gift from the Eli Lilly Co. Bathophenanthro-
line (4,7-diphenyl-l,lO-phenanthroline) was obtained from the G.
Frederick Smith Co., Columbus, Ohio. Phosphoenolpyruvate, oligo-
mycin, and lactic dehydrogenase were from Sigma. NADH and ATP
were from P-L Biochemicals.
Growth Studies-The data in Fig. 1A show the growth of
ANT-l
and oligomycin-resistant isolates on growth tubes
containing 5 pg of oligomycin/ml. Some of the isolates, such
as the ones shown in the figure, grew well at this concentration
of oligomycin and were retained for further study. Many of
the isolates failed to grow or grew poorly on growth tubes and
were discarded. An example of the properties of such isolates
will be presented in a subsequent section. Based on these
results, we retained the five oligomycin-resistant isolates
shown in Fig. 1A for further study. Fig. 1B shows the growth
rates of the five mutants and ANT-l as a function of oligo-
mycin concentration. Growth rates were determined from the
linear portion of growth curves (such as those in Fig. 1A) at
several oligomycin concentrations. The resistant strains con-
tinue to grow vigorously at concentrations of oligomycin as
high as 25 pg/ml while ANT-l failed to grow on a concentra-
tion of 0.1 pg/ml, the lowest concentration used in our studies.
Titration with Oligomycin--In order to determine whether
’ C. W. Slayman, personal communication.
the oligomycin resistance we observed was due to a loss of
permeability to the drug or to resistance of the mitochondrial
ATPase, we prepared submitochondrial particles from the
resistant isolates and carried out titration curves of ATPase
activity as a function of oligomycin concentration. Fig. 2A
shows a Dixon plot (17) of the inhibition of ATPase activity
in submitochondrial particles from ANT-l. The plot is linear,
indicating that oligomycin interacts with a single site, and it
extrapolates to give a K, for oligomycin inhibition of 0.015 pg/
mg of particle protein. Fig. 2B shows a similar plot for sub-
mitochondrial particles from oli 16-3, a representative exam-
ple of the oligomycin-resistant, isolates. The plot is again linear
and gives a K, of 2.3 pg of oligomycin/mg of particle protein.
This concentration of oligomycin is 153 times greater than
that required for 50% inhibition of ATPase activity in ANT-l.
Titration with Bathophenanthroline-Fig. 3 shows titra-
tion curves of submitochondrial particles from ANT-l and oli
16-3 with bathophenanthroline. This compound has been
shown to be an inhibitor of FI ATPase from beef heart (18,
19). It is also a potent inhibitor of the mitochondrial ATPase
activity in Neurospora. Dixon plots (17) of the inhibition
curves are again linear, indicating that bathophenanthroline
interacts with a single site or equivalent sites in these particles.
The data show that submitochondrial particles from oli 16-3
are as sensitive to inhibition by bathophenanthroline as are
the particles from ANT-l.
ATPase Parameters--When all of the mutants that we
report here were titrated with either oligomycin or batho-
phenanthroline, linear Dixon plots (17) similar to the ones in
Figs. 2 and 3 were obtained. Table I catalogues some of the
parameters that we measured in ANT-l and the mutants. The
table shows that all of the mutants studied have good levels
of ATPase activity and are highly resistant to oligomycin.
The resistances to oligomycin range from 153 to 900 times
greater than ANT-l. All of the mutants have approximately
the same sensitivity to bathophenanthroline.
All of the oligomycin-resistant strains also grow more slowly
than ANT-l. The yield of cells after 12 h of growth varies in
the mutants down to less than 20% that of ANT-l. This
suggests that the ATP synthetase activity in the mutants is
altered and that they grow slowly due to a deficiency in ATP
production. The table also shows data obtained with mutant
oli 16-15. This mutant grew poorly on growth tubes and the
data are presented as an example of the properties of those
mutants that were discarded in our screening procedure. The
data show that oli 16-15 is only moderately resistant to
oligomycin as compared with the other mutants and is also
sensitive to bathophenanthroline. These data indicate that
the growth tube studies are critical in selecting for highly
resistant mutants.
Titration with Rutamycin-The oligomycin used in these
studies was obtained commercially and is a mixture of three
isomers, oligomycin A, B, and C. We also carried out titrations
with rutamycin (oligomycin D) to determine whether differ-
ences in K, values would be obtained when a pure isomer was
used. We measured a K, for rutamycin inhibition of ANT-l
submitochondrial particles of 0.005 pg/mg. A K, of 14.0 pg/mg
was obtained with particles from oli 16-16. These results
compare favorably with the data in Table I.
Genetic AnaZysis-Table II shows the results of a random
spore analysis of crosses of the resistant mutants to ANT-l.
Each mutant was backcrossed to ANT-l using the mutant as
the protoperithecial parent. Random spores from each of the
crosses were isolated, germinated, and tested for oligomycin
sensitivity or resistance. The table shows that both oligomy-
tin-sensitive and -resistant progeny can be isolated from each
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4256 Oligomycin Resistance in Neurospora
FIB:. 1. A, growth curves of strain
ANT-l and oligomycin-resistant mu-
tants on growth tubes containing 5 pg/
ml of oligomycin. The experiments were
carried out at 30°C as described by Ro-
senberg et al. (11). The symbols used g 15
are: O-O, ANT-I; &--a, oli 16-10; =
A-A, oli 16-l; H, oli 16-14; t
S-4, oli 16-3; M, oli 16.16. B, G 10
growth rates of ANT-1 and oligomycin-
resistant mutants as a function of oligo-
mycin concentration. Growth rates were
determined from the linear portion of
growth curves for each concentration of
oligomycin. Symbols and experimental
conditions are as in A.
Time [hr]
k
.I
.5 1.0
P
5.0 10.0 25.0
Oligomycin [q/ml)
FIN;. 2. A, Dixon plot (17) of the in-
hibition of ATPase activity of submito-
chondrial particles from strain ANT-I.
Enzyme assays were carried out as de-
scribed under “Materials and Methods.”
The particles were incubated with oli-
gomycin for 5 min at 30°C before assay.
B, Dixon plot of the inhibition of ATPase
activity of submitochondrial particles
from mutant oli 16-3. Conditions are the
same as in A.
Ki=0.013 urnat /mg
BPH [pmol /mgx103]
FIN:. 3. Dixon plots of the inhibition of ATPase activity of submi-
tochondrial particles from ANT-l and oli 16-3 with bathophenan-
throline (BPH). The submitochondrial particles were incubated with
bathophenanthroline for 2 min at 30°C prior to assay. Bathophen-
anthroline was added as the tridentate chelate of ferrous iron. 0,
particles from oli 16-3; 0, particles from ANT-l.
of these crosses. In mutants oli 16-3, oli 16-14, and oli 16-16,
the numbers of resistant and sensitive progeny are roughly
equal, suggesting that these may be due to single gene muta-
tions. In mutants oli 16-l and oli 16-10, a relatively small
Oligomycin Ipg/mg]
TARIX I
Mitochondrial ATPase parameters
for
strain ANT-l and
oligomycin-resistant mutants
Conidia were inoculated at 5 x 10”/ml into Vogels’ medium N
supplemented with inositol (50 pg/ml) and sucrose (2%). The cultures
were grown at 30°C on a Gyrotory shaker for 12 h. ATPase assays
were carried out on submitochondrial particles as described under
“Materials and Methods.” Bathophenanthroline was added as the
ferrous chelate. The K, values were determined from Dixon plots (17)
of inhibition curves as shown in Figs. 1 and 2.
Strain ATPase K, oligo- K, batho-
Yield” Resistance”
activity mycin phenanthro-
line factor
g/liter ~mol/min/mg M/Jw pmol/mg
ANT-1 5.13 1.31 0.015 0.03 1.0
oli 16-1 1.33 0.86 5.50 0.04 366.7
oli 16-3 1.93 0.85 2.30 0.01 153.3
oli 16-10 1.08 1.02 6.55 0.02 436.7
oli 16-14 1.35 0.71 2.40 0.04 160.0
oli 16-16 0.98 1.60 13.50 0.05 900.0
oli 16-15 2.12 1.35 0.27 0.05 18.6
n Grams, wet weight/liter of culture medium after 12 h of growth.
’ Determined from the ratio of K, mutant to K, ANT-l.
number of resistant colonies were isolated, suggesting that
more than one nuclear gene is involved in this phenotype. A
complete genetic analysis of these mutants is presently under
way and will be published elsewhere. The data presented here,
however, are sufficient to demonstrate that mutations leading
to oligomycin resistance in Neurospora are nuclear in nature.
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Oligomycin Resistance in Neurospora 4257
TARI,F, II
Random spore analysis of crosses of oligomycin-resistant mutants
to ANT-l
Crosses were carried out on corn meal agar as described by Davis
and De Serres (16). Shot spores were collected in sterile water and
germinated by heat shock at 60°C for 45 min. The shocked spores
were plated on solid Vogels’ medium N supplemented with inositol
(50 ag/ml) and sucrose (2%). Oligomycin resistance was determined
in 1 ml of liquid medium as above supplemented with 5 pg/ml of
oligomycin. The tubes were scored after 3 days growth at 30°C. In all
cases, the percentage of spores germinating was greater than 90%.
CIVSS Oligomy-
‘Pores an- cin.resist- Oligomy- Mating type
alyzed . . cin-sensi-
ant tive A a
oli 16-IA x ANT-la 68 17 51 35 33
oli 16-3A x ANT-la 69 42 27 33 36
oli 16-IOA x ANT-la 72 8 64 42 30
oli 16-14A x ANT-la 59 34 25 29 30
oli 16-16A x ANT-la 72 25 44 28 41
DISCUSSION
The removal by genetic means of the alternative respiratory
pathways found in wild type Neurospora permits the straight-
forward selection of drug-resistant mutants that will be useful
in studies of mitochondrial biogenesis
and bioenergetics. The
data presented here provide a simple example of the potential
use of the ANT-l strain for such studies.
The mutants that we describe here are all highly resistant
to oligomycin. The resistances are increased 150- to 900-fold
over the wild type (ANT-l) level. This is in contrast to the 3-
to lo-fold increase in oligomycin resistance reported for mu-
tants obtained in S.
cereuisiae.
The increased resistance is
largely due to the fact that submitochondrial particles from
wild type
Neurospora
are much more sensitive to inhibition
by oligomycin than are particles from S.
cereuisiae.
(Compare
a
K,
for
Neurospora
of 0.015 pg/mg with an Lo of 3.0 pg/mg
for a wild type strain of yeast (20)).
The genetic analysis in Table II is sufficient to show that
all of the oligomycin-resistant mutants of
Neurospora segre-
gate in a mendelian manner in crosses and are, therefore, due
to nuclear mutations. The data suggest that some of the
mutants may be due to single gene mutations while others
may be the result of multiple mutational events. These results
are in marked contrast with those reported for S.
cereuisiae
where oligomycin resistance is conferred by at least three loci
located on mitochondrial DNA (20-22). Had the mutants we
isolated been extrachromosomal rather than nuclear, crosses
carried out in the manner reported in Table II would have
resulted in all of the progeny being oligomycin-resistant (23).
Nuclear mutations conferring oligomycin resistance have
been reported in S.
cerevisiae,
but the mitochondrial ATPase
of these strains was found to be sensitive to oligomycin (20,
24). Oligomycin resistance in these mutants is thought to be
due to impermeability to the drug.
The site of action of oligomycin has been well studied in S.
cereuisiae.
Criddle et
al.
(25) have shown that oligomycin
binds to Subunit 9 of the purified oligomycin-sensitive ATP-
ase complex and does not bind to this subunit in oligomycin-
resistant mutants. This subunit has been shown to be a
mitochondrial translation product and also to bind the ATP-
ase inhibitor dicyclohexylcarbodiimide (26, 27). The mito-
chondrial translation of this subunit is consistent with the
extrachromosomal nature of the oligomycin-resistant mutants
and it has been suggested that the mitochondrial locus
ok-1
codes for this subunit (28). In contrast to this, Jack1 and
Sebald (29) have purified the mitochondrial ATPase from
Neurospora
and have shown that the polypeptide comparable
to the yeast Subunit 9 binds dicyclohexylcarbodiimide but is
translated on cytoplasmic ribosomes (30). Whether this poly-
peptide or some other nuclear component of the ATPase
complex is involved in oligomycin binding must be determined
from further studies.
Acknowledgments-We thank Drs. Youssef Hatefi and Yves M.
Galante for helpful advice and discussions throughout the course of
this work. Dr. Galante suggested the use of ferrous bathophenanthro-
line as a common denominator for the inhibition of ATPase activities
from ANT-l and the oligomycin-resistant mutants. We also thank
Dr. Carolyn W. Slayman for providing us with the method for
preparing submitochondrial particles prior to its publication,
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
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