Abstract
Bacteria engage in contact-dependent activities to coordinate cellular activities that aid their survival. Cells of Myxococcus xanthus move over surfaces by means of type IV pili and gliding motility. Upon direct contact, cells physically exchange outer membrane (OM) lipoproteins, and this transfer can rescue motility in mutants lacking lipoproteins required for motility. The mechanism of gliding motility and its stimulation by transferred OM lipoproteins remain poorly characterized. We investigated the function of CglC, GltB, GltA and GltC, all of which are required for gliding. We demonstrate that CglC is an OM lipoprotein, GltB and GltA are integral OM β-barrel proteins, and GltC is a soluble periplasmic protein. GltB and GltA are mutually stabilizing, and both are required to stabilize GltC, whereas CglC accumulate independently of GltB, GltA and GltC. Consistently, purified GltB, GltA and GltC proteins interact in all pair-wise combinations. Using active fluorescently-tagged fusion proteins, we demonstrate that GltB, GltA and GltC are integral components of the gliding motility complex. Incorporation of GltB and GltA into this complex depends on CglC and GltC as well as on the cytoplasmic AglZ protein and the inner membrane protein AglQ, both of which are components of the gliding motility complex. Conversely, incorporation of AglZ and AglQ into the gliding motility complex depends on CglC, GltB, GltA and GltC. Remarkably, physical transfer of the OM lipoprotein CglC to a ΔcglC recipient stimulates assembly of the gliding motility complex in the recipient likely by facilitating the OM integration of GltB and GltA. These data provide evidence that the gliding motility complex in M. xanthus includes OM proteins and suggest that this complex extends from the cytoplasm across the cell envelope to the OM. These data add assembly of gliding motility complexes in M. xanthus to the growing list of contact-dependent activities in bacteria.
Highlights
Bacteria interact extensively within and between species to coordinate cellular activities or efficiently compete
We show that CglC is an outer membrane (OM) lipoprotein, GltB and GltA are integral OM β-barrel proteins, and GltC is a soluble periplasmic protein
GltB, GltA and GltC are components of the gliding motility complex, and CglC likely stimulates the integration of GltB and GltA into the OM
Summary
Bacteria interact extensively within and between species to coordinate cellular activities or efficiently compete. These interactions rely on diffusible factors or on direct cell-to-cell contacts [1,2]. Bacterial cells can move over surfaces without the aid of flagella and type IV pili by a mechanism referred to as gliding motility [9]. Bacterial flagella are homologous structures present in Gram-positive as well as–negative bacteria, mechanistically fairly well-understood and consists of four parts that together span from the cytoplasm, across the cell envelope to the cell surface [7]. Type IV pili are widespread homologous structures and the machinery underlying type IV pili function spans from the cytoplasm to the cell surface [8]. In contrast to the homologies observed for flagella and type IV pili systems, the machineries for gliding motility are nonhomologous suggesting that gliding motility has evolved independently several times [6,9] and mechanistically, gliding motility is poorly understood
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