Abstract:
Myxococcus xanthus is a gram-negative, soil bacterium that reverses its direction of movement by frequently switching the leading and lagging poles. Myxococcus xanthus motility has many similarities with the eukaryotic cell crawling machinery, i.e. small Ras-like GTPases, focal adhesion-like complexes and an actin-like protein, MreB are involved in the process. A small Ras-like GTPase, MglA and a cognate GAP (GTPase Activating Protein), MglB play significant roles in the regulation of polarity determination and motility. MglA and MglB are present at leading and lagging poles of the bacterium, respectively, and re-localize at the time of polarity reversal, i.e. they oscillate between the poles.
With the goal of understanding the molecular mechanism of polarity oscillations driven by MglA and MglB, structural and biochemical aspects of these two proteins were studied. Crystal structures of various conformational states of MglA and complexes of MglA bound to MglB have been obtained. Structure of the MglAB complex has revealed a novel allosteric interaction of C-terminal helix of one of the MglB monomers with MglA. Biochemical and mutational studies have revealed a role for the C-terminal helix in stimulating GTPase activity and in nucleotide exchange. In vivo studies in M. xanthus demonstrate severe abnormalities in motility and a bipolar localization of MglA and MglB upon deletion of the C-terminal helix from MglB, highlighting the relevance of the interaction.
A comparison of structures of the complexes of GAP and GEF (Guanine nucleotide Exchange Factor) proteins with eukaryotic small Ras-like GTPases available in the Protein Data Bank suggests that the MglAB complex structure reveals a unique mechanism of allosteric regulation of small Ras-like GTPases. Instead of interacting directly at the nucleotide binding site and exerting GAP or GEF activity as observed in majority of the structures, the C-terminal helix of MglB interacts at a binding site distal from the nucleotide binding pocket of MglA. This allosteric regulation is important for localization of proteins and polarity oscillations in M. xanthus.
