Social motility in Myxococcus xanthus requires FrzS, a protein with an extensive coiled-coil domain

Gliding motility in the developmental bacterium Myxococcus xanthus involves two genetically distinct motility systems, designated adventurous (A) and social (S). Directed motility responses, which facilitate both vegetative s warming and developmental aggregation, additionally require the 'frizzy' (F rz) signal transduction pathway. In this study, we have analysed a new gene (frzS), which is positioned upstream of the frzA-F operon. Insertion mutat ions in frzS caused both vegetative spreading and developmental defects, in cluding 'frizzy' aggregates in the FB strain background. The 'frizzy' phono type was previously considered to result only from defective directed motil ity responses. However, deletion of the frzS gene in an A(-)S(+) motility b ackground demonstrated that FrzS is a new component of the S-motility syste m, as the A(-)frzS double mutant was nonspreading (A(-)S(-)). Compared with known S-motility mutants, the frzS mutants appear similar to pilT mutants, in that both produce type IV pili, extracellular fibrils and lipopolysacch aride (LPS) O-antigen, and both agglutinate rapidly in a cohesion assay. Th e FrzS protein has an unusual domain composition for a bacterial protein. T he N-terminal domain shows similarity to the receiver domains of the two-co mponent response regulator proteins. The C-terminal domain is composed of u p to 38 heptad repeats (a b c d e f g)(38), in which residues at positions a and d are predominantly hydrophobic, whereas residues at positions e and g are predominantly charged. This periodic disposition of specific residues suggests that the domain forms a long coiled-coil structure, similar to th ose found in the alpha-fibrous proteins, such as myosin. Overexpression of this domain in Escherichia coli resulted in the formation of an unusual str iated protein lattice that filled the cells. We speculate on the role that this novel protein could play in gliding motility.