The CTPase activity of ParB determines the size and dynamics of prokaryotic DNA partition complexes.

ParB-like CTPases mediate the segregation of bacterial chromosomes and low-copy number plasmids. They act as DNA-sliding clamps that are loaded at parS motifs in the centromere of target DNA molecules and spread laterally to form large nucleoprotein complexes serving as docking points for the DNA segregation machinery. Here, we solve crystal structures of ParB in the pre- and post-hydrolysis state and illuminate the catalytic mechanism of nucleotide hydrolysis. Moreover, we identify conformational changes that underlie the CTP- and parS-dependent closure of ParB clamps. The study of CTPase-deficient ParB variants reveals that CTP hydrolysis serves to limit the sliding time of ParB clamps and thus drives the establishment of a well-defined ParB diffusion gradient across the centromere whose dynamics are critical for DNA segregation. These findings clarify the role of the ParB CTPase cycle in partition complex assembly and function and thus advance our understanding of this prototypic CTP-dependent molecular switch.

FrzS acts as a polar beacon to recruit SgmX, a central activator of type IV pili during Myxococcus xanthus motility.

In rod-shaped bacteria, type IV pili (Tfp) promote twitching motility by assembling and retracting at the cell pole. In Myxococcus xanthus, a bacterium that moves in highly coordinated cell groups, Tfp are activated by a polar activator protein, SgmX. However, while it is known that the Ras-like protein MglA is required for unipolar targeting, how SgmX accesses the cell pole to activate Tfp is unknown. Here, we demonstrate that a polar beacon protein, FrzS, recruits SgmX at the cell pole. We identified two main functional domains, including a Tfp-activating domain and a polar-binding domain. Within the latter, we show that the direct binding of MglA-GTP unveils a hidden motif that binds directly to the FrzS N-terminal response regulator (CheY). Structural analyses reveal that this binding occurs through a novel binding interface for response regulator domains. In conclusion, the findings unveil the protein interaction network leading to the spatial activation of Tfp at the cell pole. This tripartite system is at the root of complex collective behaviours in this predatory bacterium.

Cell-cell transfer of adaptation traits benefits kin and actor in a cooperative microbe.

Microbes face many physical, chemical, and biological insults from their environments. In response, cells adapt, but whether they do so cooperatively is poorly understood. Here, we use a model social bacterium, Myxococcus xanthus, to ask whether adapted traits are transferable to naïve kin. To do so we isolated cells adapted to detergent stresses and tested for trait transfer. In some cases, strain-mixing experiments increased sibling fitness by transferring adaptation traits. This cooperative behavior depended on a kin recognition system called outer membrane exchange (OME) because mutants defective in OME could not transfer adaptation traits. Strikingly, in mixed stressed populations, the transferred trait also benefited the adapted (actor) cells. This apparently occurred by alleviating a detergent-induced stress response in kin that otherwise killed actor cells. Additionally, this adaptation trait when transferred also conferred resistance against a lipoprotein toxin delivered to targeted kin. Based on these and other findings, we propose a model for stress adaptation and how OME in myxobacteria promotes cellular cooperation in response to environmental stresses.

Two reasons to kill: predation and kin discrimination in myxobacteria.

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.

Myxococcus xanthus as Host for the Production of Benzoxazoles.

Benzoxazoles are important structural motifs in pharmaceutical drugs. Here, we present the heterologous production of 3-hydroxyanthranilate-derived benzoxazoles in the host bacterium Myxococcus xanthus following the expression of two genes from the nataxazole biosynthetic gene cluster of Streptomyces sp. Tü 6176. The M. xanthus expression strain achieved a benzoxazole titer of 114.6±7.4 mg L-1 upon precursor supplementation, which is superior to other bacterial production systems. Crosstalk between the heterologously expressed benzoxazole pathway and the endogenous myxochelin pathway led to the combinatorial biosynthesis of benzoxazoles featuring a 2,3-dihydroxybenzoic acid (2,3-DHBA) building block. Subsequent in vitro studies confirmed that this crosstalk is not only due to the availability of 2,3-DHBA in M. xanthus, rather, it is promoted by the adenylating enzyme MxcE from the myxochelin pathway, which contributes to the activation of aryl carboxylic acids and delivers them to benzoxazole biosynthesis.

The polar Ras-like GTPase MglA activates type IV pilus via SgmX to enable twitching motility in Myxococcus xanthus.

Type IV pili (Tfp) are highly conserved macromolecular structures that fulfill diverse cellular functions, such as adhesion to host cells, the import of extracellular DNA, kin recognition, and cell motility (twitching). Outstandingly, twitching motility enables a poorly understood process by which highly coordinated groups of hundreds of cells move in cooperative manner, providing a basis for multicellular behaviors, such as biofilm formation. In the social bacteria Myxococcus xanthus, we know that twitching motility is under the dependence of the small GTPase MglA, but the underlying molecular mechanisms remain elusive. Here we show that MglA complexed to GTP recruits a newly characterized Tfp regulator, termed SgmX, to activate Tfp machines at the bacterial cell pole. This mechanism also ensures spatial regulation of Tfp, explaining how MglA switching provokes directional reversals. This discovery paves the way to elucidate how polar Tfp machines are regulated to coordinate multicellular movements, a conserved feature in twitching bacteria.

Myxococcus CsgA, Drosophila Sniffer, and human HSD10 are cardiolipin phospholipases.

Myxococcus xanthus development requires CsgA, a member of the short-chain alcohol dehydrogenase (SCAD) family of proteins. We show that CsgA and SocA, a protein that can replace CsgA function in vivo, oxidize the 2'-OH glycerol moiety on cardiolipin and phosphatidylglycerol to produce diacylglycerol (DAG), dihydroxyacetone, and orthophosphate. A lipid extract enriched in DAGs from wild-type cells initiates development and lipid body production in a csgA mutant to bypass the mutational block. This novel phospholipase C-like reaction is widespread. SCADs that prevent neurodegenerative disorders, such as Drosophila Sniffer and human HSD10, oxidize cardiolipin with similar kinetic parameters. HSD10 exhibits a strong preference for cardiolipin with oxidized fatty acids. This activity is inhibited in the presence of the amyloid ß peptide. Three HSD10 variants associated with neurodegenerative disorders are inactive with cardiolipin. We suggest that HSD10 protects humans from reactive oxygen species by removing damaged cardiolipin before it induces apoptosis.

Structural characterization of Myxococcus xanthus MglC, a component of the polarity control system, and its interactions with its paralog MglB.

The δ-proteobacteria Myxococcus xanthus displays social (S) and adventurous (A) motilities, which require pole-to-pole reversal of the motility regulator proteins. Mutual gliding motility protein C (MglC), a paralog of GTPase-activating protein Mutual gliding motility protein B (MglB), is a member of the polarity module involved in regulating motility. However, little is known about the structure and function of MglC. Here, we determined ∼1.85 Å resolution crystal structure of MglC using Selenomethionine Single-wavelength anomalous diffraction. The crystal structure revealed that, despite sharing <9% sequence identity, both MglB and MglC adopt a Regulatory Light Chain 7 family fold. However, MglC has a distinct ∼30° to 40° shift in the orientation of the functionally important α2 helix compared with other structural homologs. Using isothermal titration calorimetry and size-exclusion chromatography, we show that MglC binds MglB in 2:4 stoichiometry with submicromolar range dissociation constant. Using small-angle X-ray scattering and molecular docking studies, we show that the MglBC complex consists of a MglC homodimer sandwiched between two homodimers of MglB. A combination of size-exclusion chromatography and site-directed mutagenesis studies confirmed the MglBC interacting interface obtained by molecular docking studies. Finally, we show that the C-terminal region of MglB, crucial for binding its established partner MglA, is not required for binding MglC. These studies suggest that the MglB uses distinct interfaces to bind MglA and MglC. Based on these data, we propose a model suggesting a new role for MglC in polarity reversal in M. xanthus.

Characterization of glutamate-cysteine ligase and glutathione synthetase from the δ-proteobacterium Myxococcus xanthus.

Glutathione (GSH) is synthesized in two ATP-dependent reactions by glutamate-cysteine ligase (Gcl) and glutathione synthetase (Gs). Myxococcus xanthus, a gram-negative bacterium belonging to δ-proteobacteria, possesses mxGcl and mxGs, which have high sequence identity with the enzymes from plants and bacteria, respectively. MxGcl2 was activated by Mn2+ , but not by Mg2+ , and stabilized in the presence of 5 mM Mn2+ or Mg2+ . Sequence comparison of mxGcl2 and Brassica juncea Gcl indicated that they have the same active site residues, except for Tyr330, which interacts with Cys and which in mxGcl2 is represented by Leu267. The substitution of Leu267 with Tyr resulted in the loss of mxGcl2 activity, but that with Met (found in cyanobacterial Gcls) increased the mxGcl2 affinity for Cys. GSH and its oxidized form GSSG equally inhibited the activity of mxGcl2; the inhibition was augmented by ATP at concentrations >3 mM. Buthionine sulfoximine inactivated mxGcl2 with Ki  = 2.1 µM, which was lower than those for Gcls from other organisms. The mxGcl2 activity was also suppressed by pyrophosphate and polyphosphates. MxGs was a dimer, and its activity was induced by Mg2+ but strongly inhibited by Mn2+ even in the presence of 10 mM Mg2+ . MxGs was inhibited by GSSG at Ki  = 3.6 mM. Approximately 1 mM GSH was generated with 3 units of mxGcl2 and 6 units of mxGs from 5 mM Glu, Cys, and Gly, and 10 mM ATP. Our results suggest that GSH production in M. xanthus mostly depends on mxGcl2 activity.

Spatiotemporal distribution of chemical signatures exhibited by Myxococcus xanthus in response to metabolic conditions.

Myxococcus xanthus is a common soil bacterium with a complex life cycle, which is known for production of secondary metabolites. However, little is known about the effects of nutrient availability on M. xanthus metabolite production. In this study, we utilize confocal Raman microscopy (CRM) to examine the spatiotemporal distribution of chemical signatures secreted by M. xanthus and their response to varied nutrient availability. Ten distinct spectral features are observed by CRM from M. xanthus grown on nutrient-rich medium. However, when M. xanthus is constrained to grow under nutrient-limited conditions, by starving it of casitone, it develops fruiting bodies, and the accompanying Raman microspectra are dramatically altered. The reduced metabolic state engendered by the absence of casitone in the medium is associated with reduced, or completely eliminated, features at 1140 cm-1, 1560 cm-1, and 1648 cm-1. In their place, a feature at 1537 cm-1 is observed, this feature being tentatively assigned to a transitional phase important for cellular adaptation to varying environmental conditions. In addition, correlating principal component analysis heat maps with optical images illustrates how fruiting bodies in the center co-exist with motile cells at the colony edge. While the metabolites responsible for these Raman features are not completely identified, three M. xanthus peaks at 1004, 1151, and 1510 cm-1 are consistent with the production of lycopene. Thus, a combination of CRM imaging and PCA enables the spatial mapping of spectral signatures of secreted factors from M. xanthus and their correlation with metabolic conditions.

The Fluidity of the Bacterial Outer Membrane Is Species Specific: Bacterial Lifestyles and the Emergence of a Fluid Outer Membrane.

The outer membrane (OM) is an essential barrier that guards Gram-negative bacteria from diverse environmental insults. Besides functioning as a chemical gatekeeper, the OM also contributes towards the strength and stiffness of cells and allows them to sustain mechanical stress. Largely influenced by studies of Escherichia coli, the OM is viewed as a rigid barrier where OM proteins and lipopolysaccharides display restricted mobility. Here the discussion is extended to other bacterial species, with a focus on Myxococcus xanthus. In contrast to the rigid OM paradigm, myxobacteria possess a relatively fluid OM. It is concluded that the fluidity of the OM varies across environmental species, which is likely linked to their evolution and adaptation to specific ecological niches. Importantly, a fluid OM can endow bacteria with distinct functions for cell-cell and cell-environment interactions.

Profiling Myxococcus xanthus Swarming Phenotypes through Mutation and Environmental Variation.

Myxococcus xanthus is a bacterium that lives on surfaces as a predatory biofilm called a swarm. As a growing swarm feeds on prey and expands, it displays dynamic multicellular patterns such as traveling waves called ripples and branching protrusions called flares. The rate at which a swarm expands across a surface, and the emergence of the coexisting patterns, are all controlled through coordinated cell movement. M. xanthus cells move using two motility systems known as adventurous (A) and social (S). Both are involved in swarm expansion and pattern formation. In this study, we describe a set of M. xanthus swarming genotype-to-phenotype associations that include both genetic and environmental perturbations. We identified new features of the swarming phenotype, recorded and measured swarm expansion using time-lapse microscopy, and compared the impact of mutations on different surfaces. These observations and analyses have increased our ability to discriminate between swarming phenotypes and provided context that allows us to identify some phenotypes as improbable outliers within the M. xanthus swarming phenome. IMPORTANCE Myxococcus xanthus grows on surfaces as a predatory biofilm called a swarm. In nature, a feeding swarm expands by moving over and consuming prey bacteria. In the laboratory, a swarm is created by spotting cell suspension onto nutrient agar in lieu of prey. The suspended cells quickly settle on the surface as the liquid is absorbed into the agar, and the new swarm then expands radially. An assay that measures the expansion rate of a swarm of mutant cells is the first, and sometimes only, measurement used to decide whether a particular mutation impacts swarm motility. We have broadened the scope of this assay by increasing the accuracy of measurements and introducing prey, resulting in new identifiable and quantifiable features that can be used to improve genotype-to-phenotype associations.

Dynamical patterning modules and network motifs as joint determinants of development: Lessons from an aggregative bacterium.

Development and evolution are dynamical processes under the continuous control of organismic and environmental factors. Generic physical processes, associated with biological materials and certain genes or molecules, provide a morphological template for the evolution and development of organism forms. Generic dynamical behaviors, associated with recurring network motifs, provide a temporal template for the regulation and coordination of biological processes. The role of generic physical processes and their associated molecules in development is the topic of the dynamical patterning module (DPM) framework. The role of generic dynamical behaviors in biological regulation is studied via the identification of the associated network motifs (NMs). We propose a joint DPM-NM perspective on the emergence and regulation of multicellularity focusing on a multicellular aggregative bacterium, Myxococcus xanthus. Understanding M. xanthus development as a dynamical process embedded in a physical substrate provides novel insights into the interaction between developmental regulatory networks and generic physical processes in the evolutionary transition to multicellularity.

ImuA Facilitates SOS Mutagenesis by Inhibiting RecA-Mediated Activity in Myxococcus xanthus.

Bacteria have two pathways to restart stalled replication forks caused by environmental stresses, error-prone translesion DNA synthesis (TLS) catalyzed by TLS polymerase and error-free template switching catalyzed by RecA, and their competition on the arrested fork affects bacterial SOS mutagenesis. DnaE2 is an error-prone TLS polymerase, and its functions require ImuA and ImuB. Here, we investigated the transcription of imuA, imuB, and dnaE2 in UV-C-irradiated Myxococcus xanthus and found that the induction of imuA occurred significantly earlier than that of the other two genes. Mutant analysis showed that unlike that of imuB or dnaE2, the deletion of imuA significantly delayed bacterial regrowth and slightly reduced the bacterial mutation frequency and UV resistance. Transcriptomic analysis revealed that the absence of ImuA released the expression of some known SOS genes, including recA1, recA2, imuB, and dnaE2. Yeast two-hybrid and pulldown analyses proved that ImuA interacts physically with RecA1 besides ImuB. Protein activity analysis indicated that ImuA had no DNA-binding activity but inhibited the DNA-binding and recombinase activity of RecA1. These findings indicate the new role of ImuA in SOS mutagenesis; that is, ImuA inhibits the recombinase activity of RecA1, thereby facilitating SOS mutagenesis in M. xanthus. IMPORTANCE DnaE2 is responsible for bacterial SOS mutagenesis in nearly one-third of sequenced bacterial strains. However, its mechanism, especially the function of one of its accessory proteins, ImuA, is still unclear. Here, we report that M. xanthus ImuA could affect SOS mutagenesis by inhibiting the recombinase activity of RecA1, which helps to explain the mechanism of DnaE2-dependent TLS and the selection of the two restart pathways to repair the stalled replication fork.

Two PAAR Proteins with Different C-Terminal Extended Domains Have Distinct Ecological Functions in Myxococcus xanthus.

Bacterial proline-alanine-alanine-arginine (PAAR) proteins are located at the top of the type VI secretion system (T6SS) nanomachine and carry and deliver effectors into neighboring cells. Many PAAR proteins are fused with a variable C-terminal extended domain (CTD). Here, we report that two paar-ctd genes (MXAN_RS08765 and MXAN_RS36995) located in two homologous operons are involved in different ecological functions of Myxococcus xanthusMXAN_RS08765 inhibited the growth of plant-pathogenic fungi, while MXAN_RS36995 was associated with the colony-merger incompatibility of M. xanthus cells. These two PAAR-CTD proteins were both toxic to Escherichia coli cells, while MXAN_RS08765, but not MXAN_RS36995, was also toxic to Saccharomyces cerevisiae cells. Their downstream adjacent genes, i.e., MXAN_RS08760 and MXAN_RS24590, protected against the toxicities. The MXAN_RS36995 protein was demonstrated to have nuclease activity, and the activity was inhibited by the presence of MXAN_RS24590. Our results highlight that the PAAR proteins diversify the CTDs to play divergent roles in M. xanthusIMPORTANCE The type VI secretion system (T6SS) is a bacterial cell contact-dependent weapon capable of delivering protein effectors into neighboring cells. The PAAR protein is located at the top of the nanomachine and carries an effector for delivery. Many PAAR proteins are extended with a diverse C-terminal sequence with an unknown structure and function. Here, we report two paar-ctd genes located in two homologous operons involved in different ecological functions of Myxococcus xanthus; one has antifungal activity, and the other is associated with the kin discrimination phenotype. The PAAR-CTD proteins and the proteins encoded by their downstream genes form two toxin-immunity protein pairs. We demonstrated that the C-terminal diversification of the PAAR-CTD proteins enriches the ecological functions of bacterial cells.

Is "Wolf-Pack" Predation by Antimicrobial Bacteria Cooperative? Cell Behaviour and Predatory Mechanisms Indicate Profound Selfishness, Even when Working Alongside Kin.

For decades, myxobacteria have been spotlighted as exemplars of social "wolf-pack" predation, communally secreting antimicrobial substances into the shared public milieu. This behavior has been described as cooperative, becoming more efficient if performed by more cells. However, laboratory evidence for cooperativity is limited and of little relevance to predation in a natural setting. In contrast, there is accumulating evidence for predatory mechanisms promoting "selfish" behavior during predation, which together with conflicting definitions of cooperativity, casts doubt on whether microbial "wolf-pack" predation really is cooperative. Here, it is hypothesized that public-goods-mediated predation is not cooperative, and it is argued that a holistic model of microbial predation is needed, accounting for predator and prey relatedness, social phenotypes, spatial organization, activity/specificity/transport of secreted toxins, and prey resistance mechanisms. Filling such gaps in our knowledge is vital if the evolutionary benefits of potentially costly microbial behaviors mediated by public goods are to be properly understood.

Biocontrol mechanism of Myxococcus xanthus B25-I-1 against Phytophthora infestans.

Phytophthora infestans is the pathogen causing potato late blight, one of the most serious diseases of potato. Myxobacteria have become a valuable biological control resource due to their preponderant abilities to produce various secondary metabolites with novel structure and remarkable biological activity. In this study, Myxococcus xanthus strain B25-I-1, which exhibited strong antagonistic activity against P. infestans, was isolated from soil sample and identified by 16S rRNA sequence analysis. The strain exhibited antagonistic activity against several species of fungus and bacteria. Analysis of the biocontrol mechanism showed that the active extract produced by strain B25-I-1 had strong inhibitory effects on mycelium and the asexual and sexual reproductive structures of P. infestans. Furthermore, these active extract decreased the content of soluble proteins and activity of the protective enzymes (PPO, POD, PAL, and SOD), increased the oxidative damage and the permeability of the cell membrane in P. infestans. All of these mechanisms might be the biocontrol mechanism of B25-I-1 against P. infestans. The active extract of strain B25-I-1 was separated by TLC and HPLC, and the components with antibiotic activity were detected by HPLC-MS. It was found that the antagonistic components of B25-I-1 contained methyl (2R)-2-azido-3-hydroxyl-2-methylpropanoate and N-(3-Amino-2-hydroxypropyl)-N-methylsulfuric diamide. The active extract significantly inhibited the infection on detached potato leaves by P. infestans, and these substances did not cause damage to the potato leaves. In conclusion, M. xanthus B25-I-1 produced active extract against P. infestans and might potentially be a candidate to develop into biological pesticides for the control of potato late blight. This study adds to the literature on the isolation and identification of active extracts from myxobacteria, and B25-I-1 in particular, for cures or treatments to potato late blight.

Characterization of the Exopolysaccharide Biosynthesis Pathway in Myxococcus xanthus.

Myxococcus xanthus arranges into two morphologically distinct biofilms depending on its nutritional status, i.e., coordinately spreading colonies in the presence of nutrients and spore-filled fruiting bodies in the absence of nutrients. A secreted polysaccharide, referred to as exopolysaccharide (EPS), is a structural component of both biofilms and is also important for type IV pilus-dependent motility and fruiting body formation. Here, we characterize the biosynthetic machinery responsible for EPS biosynthesis using bioinformatics, genetics, heterologous expression, and biochemical experiments. We show that this machinery constitutes a Wzx/Wzy-dependent pathway dedicated to EPS biosynthesis. Our data support that EpsZ (MXAN_7415) is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for the initiation of the repeat unit synthesis. Heterologous expression experiments support that EpsZ has galactose-1-P transferase activity. Moreover, MXAN_7416, renamed WzxEPS, and MXAN_7442, renamed WzyEPS, are the Wzx flippase and Wzy polymerase responsible for translocation and polymerization of the EPS repeat unit, respectively. In this pathway, EpsV (MXAN_7421) also is the polysaccharide copolymerase and EpsY (MXAN_7417) the outer membrane polysaccharide export (OPX) protein. Mutants with single in-frame deletions in the five corresponding genes had defects in type IV pilus-dependent motility and a conditional defect in fruiting body formation. Furthermore, all five mutants were deficient in type IV pilus formation, and genetic analyses suggest that EPS and/or the EPS biosynthetic machinery stimulates type IV pilus extension. Additionally, we identify a polysaccharide biosynthesis gene cluster, which together with an orphan gene encoding an OPX protein make up a complete Wzx/Wzy-dependent pathway for synthesis of an unknown polysaccharide.IMPORTANCE The secreted polysaccharide referred to as exopolysaccharide (EPS) has important functions in the social life cycle of M. xanthus; however, little is known about how EPS is synthesized. Here, we characterized the EPS biosynthetic machinery and showed that it makes up a Wzx/Wzy-dependent pathway for polysaccharide biosynthesis. Mutants lacking a component of this pathway had reduced type IV pilus-dependent motility and a conditional defect in development. These analyses also suggest that EPS and/or the EPS biosynthetic machinery is important for type IV pilus formation.

Highly Signal-Responsive Gene Regulatory Network Governing Myxococcus Development.

The bacterium Myxococcus xanthus undergoes multicellular development when starved. Thousands of cells build mounds in which some differentiate into spores. This remarkable feat and the genetic tractability of Myxococcus provide a unique opportunity to understand the evolution of gene regulatory networks (GRNs). Recent work has revealed a GRN involving interconnected cascades of signal-responsive transcriptional activators. Initially, starvation-induced intracellular signals direct changes in gene expression. Subsequently, self-generated extracellular signals provide morphological cues that regulate certain transcriptional activators. However, signals for many of the activators remain to be discovered. A key insight is that activators often work combinatorially, allowing signal integration. The Myxococcus GRN differs strikingly from those governing sporulation of Bacillus and Streptomyces, suggesting that Myxococcus evolved a highly signal-responsive GRN to enable complex multicellular development.

Conditional requirement of SglT for type IV pili function and S-motility in Myxococcus xanthus.

Myxobacteria exhibit complex social behaviors such as predation, outer membrane exchange and fruiting body formation. These behaviors depend on coordinated movements of cells on solid surfaces that involve social (S) motility. S-motility is powered by extension-retraction cycles of type 4 pili (Tfp) and exopolysaccharides (EPS) that provide a matrix for group cellular movement. Here, we characterized a new class of S-motility mutants in Myxococcus xanthus. These mutants have a distinctive phenotype: they lack S-motility even though they produce pili and EPS and the phenotype is temperature-sensitive. The point mutations were mapped to a single locus, MXAN_3284, named sglT. Similar to pilT mutants, sglT mutants are hyperpiliated and, strikingly, the temperature-sensitive phenotype is caused by null mutations. Our results indicate that SglT plays a critical role in Tfp function associated with pilus retraction and that the block in pili retraction is caused by a Tfp assembly defect in the absence of SglT at high-temperature growth.