SipC multimerization promotes actin nucleation and contributes to Salmonella-induced inflammation

Actin nucleation is the rate-limiting step in actin assembly and is regulated by actin-binding proteins and signal transduction molecules. Salmonella enterica serovar Typhimurium exploits actin dynamics by reorganizing the host actin cytoskeleton to facilitate its own uptake. SipC is a Salmonella actin-binding protein that nucleates actin filament formation in vitro. The molecular mechanism by which SipC nucleates actin is not known. We show here that SipC(199-409) forms multimers to promote actin nucleation. We found that wild-type SipC(199-409) forms dimers and multimers while SipC(199-409)#1, a nucleation mutant, is less efficient in dimer and multimer formation. Biochemical analysis suggested that SipC(199-409) might form parallel dimers in an extended conformation. Furthermore, a mutant Salmonella strain that was defective in forming the SipC multimer and deficient in actin nucleation failed to cause severe colitis in a mouse model. These results allow us to present a model in which SipC forms multimers to promote actin nucleation.