Figure - available from: Molecular Microbiology
This content is subject to copyright. Terms and conditions apply.
MYXO‐CTERM is a universal motif in the order Myxococcales. Four different WebLogos show the MYXO‐CTERM motif from three defined suborders and an unclassified (catchall) suborder. Colored triangles indicate the number of proteins binned into each suborder
Source publication
Cells interact with their surrounding environment through surface proteins. However, knowledge gaps remain in understanding how these important types of proteins are transported and anchored on the cell surface. In the Gram‐negative social bacterium, Myxococcus xanthus, a putative C‐terminal sorting tag (MYXO‐CTERM) is predicted to help direct 34 d...
Similar publications
Myxobacteria are part of the phylum Myxococcota, encompassing four orders. Most of them display complex lifestyles and broad predation profiles. However, metabolic potential and predation mechanisms of different myxobacteria remains poorly understood. Herein, we used comparative genomics and transcriptomics to analyze metabolic potentials and diffe...
Citations
... MYXO-CTERM appears on over 30 proteins in the Myxococcota species Myxococcus xanthus, including the TraA protein later shown to be involved in the sharing of outer membrane proteins and lipids by compatible strains (7). As with rhombosortase substrates, MYXO-CTERM proteins likewise require processing by a T2SS system to reach the outer leaflet of the outer membrane (14). ...
... The similarity of the JDVT-CTERM domain to all three other known (14) or proposed sorting signals in our collection that feature invariant Cys residues, including MYXO-CTERM, suggests that all four putative protein-sorting domains would have processing enzymes from the same superfamily with the same general mechanism. Bioinformatic exploration of evidence supporting the involvement of CPBP family enzymes in the other three systems would therefore be a good test of our proposal. ...
... Nine of these 12 belong to a cassette that encodes an apparently divergent subclass of the T2SS operon. This strongly suggests that PPP is giving a meaningful signal, since both GlyGly-CTERM-tagged proteins processed by rhombosortase in Vibrio cholerae (12) and MYXO-CTERM proteins sorted by the missing myxosortase of Myxococcus xanthus (14) were shown to require a T2SS to complete the process of movement across the outer membrane and attachment on the cell surface. The most sharply divergent subunits of the Synerg-CTERM-associated subclass of T2SS apparatus, and the HMMs built to describe them, included GspC (NF041616), GspL (NF041617), GspM (NF041618), and GspN (NF041619). ...
The LPXTG protein-sorting signal, found in surface proteins of various Gram-positive pathogens, was the founding member of a growing panel of prokaryotic small C-terminal sorting domains. Sortase A cleaves LPXTG, exosortases (XrtA and XrtB) cleave the PEP-CTERM sorting signal, archaeosortase A cleaves PGF-CTERM, and rhombosortase cleaves GlyGly-CTERM domains. Four sorting signal domains without previously known processing proteases are the MYXO-CTERM, JDVT-CTERM, Synerg-CTERM, and CGP-CTERM domains. These exhibit the standard tripartite architecture of a short signature motif, a hydrophobic transmembrane segment, and an Arg-rich cluster. Each has an invariant cysteine in its signature motif. Computational evidence strongly suggests that each of these four Cys-containing sorting signals is processed, at least in part, by a cognate family of glutamic-type intramembrane endopeptidases related to the eukaryotic type II CAAX-processing protease Rce1. For the MYXO-CTERM sorting signals of different lineages, their sorting enzymes, called myxosortases, include MrtX (MXAN_2755 in Myxococcus xanthus ), MrtC, and MrtP, all with radically different N-terminal domains but with a conserved core. Related predicted sorting enzymes were also identified for JDVT-CTERM (MrtJ), Synerg-CTERM (MrtS), and CGP-CTERM (MrtA). This work establishes a major new family of protein-sorting housekeeping endopeptidases contributing to the surface attachment of proteins in prokaryotes.
IMPORTANCE
Homologs of the eukaryotic type II CAAX-box protease Rce1, a membrane-embedded endopeptidase found in yeast and human ER and involved in sorting proteins to their proper cellular locations, are abundant in prokaryotes but not well understood there. This bioinformatics paper identifies several subgroups of the family as cognate endopeptidases for four protein-sorting signals processed by previously unknown machinery. Sorting signals with newly identified processing enzymes include three novel ones, but also MYXO-CTERM, which had been the focus of previous experimental work in the model fruiting and gliding bacterium Myxococcus xanthus . The new findings will substantially improve our understanding of Cys-containing C-terminal protein-sorting signals and of protein trafficking generally in bacteria and archaea.
... The variable domain provides specificity for homotypic TraA-TraA binding between cells. Full length proteins contain ~80 cysteine residues where the mature C-terminal residue shown is a cysteine that is predicted to be lipidated, which thus anchors TraA on the cell surface [102]. TraB is an OM β-barrel protein that interacts with TraA and for simplicity is not shown. ...
Myxobacteria are social microbial predators that use cell-cell contacts to identify bacterial or fungal prey and to differentiate kin relatives to initiate cellular responses. For prey killing, they assemble Tad-like and type III-like secretion systems at contact sites. For kin discrimination (KD), they assemble outer membrane exchange complexes composed of the TraA and TraB receptors at contacts sites. A type VI secretion system and Rhs proteins also mediate KD. Following cellular recognition, these systems deliver appropriate effectors into target cells. For prey, this leads to cell death and lysis for nutrient consumption by myxobacteria. In KD, a panel of effectors are delivered, and if adjacent cells are clonal cells, resistance ensues because they express a cognate panel of immunity factors; while nonkin lack complete immunity and are intoxicated. This review compares and contrasts recent findings from these systems in myxobacteria.
... The tested M. xanthus mutant strains displayed normal swarm expansion when applied to nutrient agar plates with 1% casitone, confirming that reduced prey clearing is not caused by a defect in cell motility ( Figure S4A). We did not analyze mutants of the main T2SS cluster MXAN_2405-2515 (general secretory pathway), as we were not successful in constructing mutants in this essential system (data not shown; Konovalova et al., 2009;Sah et al., 2020). ...
Predatory Myxobacteria employ a multilayered predation strategy to kill and lyse soil microorganisms. Aiming to dissect the mechanism of contact-dependent killing of bacteria, we analyze four protein secretion systems in Myxococcus xanthus and investigate the predation of mutant strains on different timescales. We find that a Tad-like and a type 3-like secretion system (Tad and T3SS∗) fulfill distinct functions during contact-dependent prey killing: the Tad-like system is necessary to induce prey cell death, while the needle-less T3SS∗ initiates prey lysis. Fluorescence microscopy reveals that components of both systems interdependently localize to the predator-prey contact site prior to killing. Swarm expansion assays show that both Tad and T3SS∗ are required to handle live prey and that nutrient extraction from prey bacteria is sufficient to power M. xanthus motility. In conclusion, our observations indicate the functional interplay of two types of secretion systems for killing and lysis of bacterial cells.
... This might be required because membrane fusion requires energy, and in the OM, such energy could be derived from irreversible conformational changes in TraAB. In contrast, in the latter scenario, TraC may be required for efficient posttranslational processing, folding, and/or secretion of TraAB to the cell surface (51). Consistent with this, the top BLAST hit for TraC is the PEP_CTERM TPR lipoprotein (TIGR02917), thought to be involved in anchoring proteins to the cell surface with C-terminal tags (52). ...
... Consistent with this, the top BLAST hit for TraC is the PEP_CTERM TPR lipoprotein (TIGR02917), thought to be involved in anchoring proteins to the cell surface with C-terminal tags (52). This homology is intriguing since TraA contains an analogous MYXO-CTERM motif at its C-terminal end (51). In either of these scenarios, this lack of OME activity in a DtraC mutant, while TraAB levels are actually elevated, could be explained by a fraction of the TraAB protein pool being nonfunctional and perhaps even eliciting a dominant negative phenotype. ...
... Western blotting and fluorescence microscopy. Western blot assays were done essentially as described previously (51). Primary polyclonal rabbit antibodies were anti-GFP (1:15,000 dilution; Invitrogen), anti-FLAG (1:1,500 dilution; Sigma), anti-TraA (1:25,000 dilution) (11), and anti-TraB (1:15,000 dilution) (9). ...
Bacteria compete against related individuals by delivering toxins. In myxobacteria, a key delivery and kin discrimination mechanism is called outer membrane (OM) exchange (OME). Here, cells that display compatible polymorphic cell surface receptors recognize one another and bidirectionally transfer OM content. Included in the cargo is a suite of polymorphic SitA lipoprotein toxins. Consequently, OME between compatible cells that are not clonemates results in intoxication, while exchange between clonemates is harmonious because cells express a cognate repertoire of immunity proteins, which themselves are not transferred. SitA toxins belong to six nonhomologous families classified by sequence conservation within their N-terminal “escort domains” (EDs), while their C termini contain polymorphic nucleases that target the cytoplasmic compartment. To investigate how toxins delivered to the OM by OME translocate to the cytoplasm, we selected transposon mutants resistant to each family. Our screens identified eight genes that conferred resistance in a SitA family-specific manner. Most of these genes are predicted to localize to the cell envelope, and some resemble proteins that colicins exploit to gain cell entry. By constructing functional chimeric SitAs between families, we show that the ED determines the specificity of resistance. Importantly, a mutant that confers resistance to all six SitA families was discovered. This gene was named traC and plays an accessory role with traAB in OME. This work thus provides insight into the mechanism of kin discrimination in myxobacteria and provides working models for how SitA toxins exploit host proteins to gain cytoplasmic entry.
The LPXTG protein-sorting signal, found in surface proteins of various Gram-positive pathogens, was the founding member of a growing panel of prokaryotic small C-terminal sorting domains. Sortase A (SrtA) cleaves LPXTG, exosortases (XrtA and XrtB) cleave the PEP-CTERM sorting signal, archaeosortase A (ArtA) cleaves PGF-CTERM, and rhombosortase (RrtA) cleaves GlyGly-CTERM domains. Four sorting signal domains without previously known processing proteases are the MYXO-CTERM, JDVT-CTERM, Synerg-CTERM, and CGP-CTERM domains. These exhibit the standard tripartite architecture of short signature motif, then a hydrophobic transmembrane segment, then an Arg-rich cluster. Each has an invariant cysteine in its signature motif. Computational evidence strongly suggests that each of these four Cys-containing sorting signals is processed, at least in part, by a cognate family of glutamic-type intramembrane endopeptidases, related to eukaryotic type II CAAX-processing protease Rce1. For the MYXO-CTERM sorting signals of different lineages, their sorting enzymes, called myxosortases, include MrtX (MXAN_2755 in Myxococcus xanthus ), MrtC, and MrtP, all with radically different N-terminal domains but with a conserved core. Predicted cognate sorting enzymes were identified also for JDVT-CTERM (MrtJ), Synerg-CTERM (MrtS), and CGP-CTERM (MrtA). This work establishes a major new family of protein-sorting housekeeping endopeptidases contributing to surface attachment of proteins in prokaryotes.
Importance
Homologs of the eukaryotic type II CAAX-box protease Rce1, a membrane-embedded endopeptidase found in yeast and human ER and involved in sorting proteins to their proper cellular locations, are abundant in prokaryotes but are not well understood there. This bioinformatics paper identifies several subgroups of the family as cognate endopeptidases for four protein-sorting signals processed by previously unknown machinery. Sorting signals with newly identified processing enzymes include three novel ones, but also MYXO-CTERM, which had been the focus of previous experimental work in the model fruiting and gliding bacterium Myxococcus xanthus . The new findings will substantially improve our understanding of Cys-containing C-terminal protein-sorting signals and of protein trafficking generally in bacteria and archaea.
A wide range of biological systems – from microbial swarms to bird flocks, display emergent behaviors driven by coordinated movement of individuals. To this end, individual organisms interact by recognizing their kin and adjusting their motility based on others around them. However, even in the best-studied systems, the mechanistic basis of the interplay between kin recognition and motility coordination is not understood. Here, using a combination of experiments and mathematical modeling, we uncover the mechanism of an emergent social behavior in Myxococcus xanthus . By overexpressing the cell surface kin recognition receptors, TraA and TraB, large numbers of cells adhere to one another and form organized macroscopic circular aggregates that spin clockwise or counterclockwise. Mechanistically, TraAB adhesion results in sustained cell-cell contacts that signal the Frz chemosensory pathway. In turn, cell reversals are suppressed and circular aggregates form as the result of cells’ ability to follow cellular slime trails. Furthermore, our in-silico simulations demonstrate a remarkable ability to predict self-organization patterns when phenotypically distinct strains are mixed. Therefore, this work provides key mechanistic insights into M. xanthus social interactions and demonstrates how social recognition transforms physical interactions into emergent collective behaviors.
Significance
In many species, large populations exhibit emergent behaviors whereby all related individuals move in unison. For example, fish in schools can all dart in one direction simultaneously avoiding a predator. Currently, it is impossible to explain how such animals recognize kin and elicit these behaviors at a molecular level. However, microbes also recognize kin and exhibit emergent collective behaviors that are experimentally tractable. Here, using a model social bacterium, we engineer dispersed individuals to organize into synchronized collectives that create emergent patterns. With experimental and mathematical approaches we explain how this occurs at both molecular and population levels. The results demonstrate how the combination of local physical interactions and signaling between cells leads to emergent behavior on a global scale.
Myxobacteria conduct complex social traits that requires populations to be highly related and devoid of exploiters. To enrich for clonal cells in populations, they employ kin discrimination mechanisms. One key system involves a polymorphic cell surface receptor, TraA, which recognizes self by homotypic interactions with neighboring myxobacterial cells. Recent studies revealed that TraA and its partner TraB are fluid outer membrane proteins that coalesce into foci upon recognition of kin. The formation of foci leads to transient membrane fusion junctions and the bidirectional exchange of outer membrane components that facilitates cooperative behaviors. Additionally, expansive suites of polymorphic lipoprotein toxins are exchanged, which act as self-identity barcodes that exquisitely discriminate against nonself to assemble homogenous populations.
