A proposed chaperone of the bacterial type VI secretion system functions to constrain a self-identity protein

Abstract

The opportunistic bacterial uropathogen Proteus mirabilis can communicate identity through the export of the self-identity protein, IdsD, via the type VI secretion (T6S) system. Expression of the ids genes provides a fitness advantage during polymicrobial infections in a mouse infection model. Here we provide an answer to the unresolved question of how the activity of a T6S substrate, such as IdsD, is regulated before export. We demonstrate that IdsD is found in clusters that form independently of the T6S machinery and activity. We show that the protein IdsC, which is a member of the proposed DUF4123 chaperone family, is essential for the stability of these clusters as well as the IdsD protein itself. And we provide evidence that amino acid disruptions in IdsC are sufficient to disrupt IdsD export but not IdsD localization into stable subcellular clusters, strongly supporting that IdsC functions in at least two different ways: IdsD stabilization and IdsD export. We propose that IdsC, and likely other DUF4123-containing proteins, function to regulate T6S substrates before export by both stabilizing the protein and mediating export at the T6S machinery.