ATPase-Independent Type-III Protein Secretion in Salmonella enterica

Abstract

Type-III protein secretion systems are utilized by gram-negative pathogens to secrete building blocks of the bacterial flagellum, virulence effectors from the cytoplasm into host cells, and structural subunits of the needle complex. The flagellar type-III secretion apparatus utilizes both the energy of the proton motive force and ATP hydrolysis to energize substrate unfolding and translocation. We report formation of functional flagella in the absence of type-III ATPase activity by mutations that increased the proton motive force and flagellar substrate levels. We additionally show that increased proton motive force bypassed the requirement of the Salmonella pathogenicity island 1 virulence-associated type-III ATPase for secretion. Our data support a role for type-III ATPases in enhancing secretion efficiency under limited secretion substrate concentrations and reveal the dispensability of ATPase activity in the type-III protein export process.

Author Summary

The type-III protein secretion apparatus is a complex nanomachine responsible for secretion of building blocks and substrate proteins of the flagellum and the virulence-associated injectisome needle complex of many gram-negative pathogens. Type-III secretion systems utilize the energy of the proton motive force and ATP hydrolysis of a cytoplasmic ATPase to drive substrate export. The cytoplasmic components of the secretion system share strong homology to the F _oF ₁ ATP synthase and it is thought that the flagellum was derived from a proto F _oF ₁-ATP synthase where ATP hydrolysis energized the export process. Here, we report the dispensability of ATPase activity for the type-III protein export process in Salmonella. This finding has important implications for the evolution of the bacterial flagellum and type-III secretion systems, suggesting that a proto ATPase was added to a primordial proton-powered type-III export system with the evolutionary benefit of facilitating the export process.