The Salmonella FlgA protein, a putative periplasmic chaperone essential for flagellar P ring formation

P ring is a periplasmic substructure of the flagellar basal body and is believed to connect with the peptidoglycan layer in Salmonella. Two flagellar genes, flgA and flgl, are known to be indispensable for P ring formation. The flgl gene encodes the component protein of the P ring. However, the role of the flgA gene product in P ring assembly remained unknown. Here, evidence is presented that FlgA is synthesized as a precursor form and exported via the Sec secretory pathway into the periplasmic space where P ring formation takes place. Overproduction of the Flgl protein led flgA mutants to form flagella with a P ring, suggesting that FlgA plays an auxiliary role in P ring assembly. Far-Western blot analysis revealed that FlgA binds in vitro to both Flgl and FlgA itself. Though a direct Flgl-Flgl interaction in the absence of FlgA could not be demonstrated, an indirect or direct interaction between the Flgl proteins was observed in the presence of FlgA. FlgA alone was very unstable in vivo, but co-expression with Flgl could stabilize FlgA. This suggests the presence of FlgA-Flgl interaction in vivo. On the basis of these results, a hypothesis is proposed that FlgA acts as a periplasmic chaperone, which assists a polymerization reaction of Flgl into the P ring through FlgA-Flgl interaction.