THE STRUCTURE OF A PUTATIVE TYPE VIIb SECRETION SYSTEM CHAPERONE PROTEIN FAMILY

Abstract

Bacteria interact with their environment using sophisticated protein export pathways. The type VII secretion system (T7SS) is a membrane-bound ATP- dependent protein translocase found throughout the Gram-positive phyla Actinobacteria and Firmicutes, where it is referred to as T7SSa and T7SSb, respectively. Most studies on the T7SSb system focus on its role in bacterial competition. The T7SSb secretes polymorphic antibacterial toxins that contain an α-helical domain belonging to the Leucine-X-Glycine (LXG) protein family. LXG proteins are genetically encoded next to two small LXG-associated α-helical proteins (Laps). We have recently demonstrated that Laps form a heterotrimeric complex with their cognate T7SS toxin. However, the molecular details governing LXG toxin recognition and recruitment to the T7SSb apparatus remain incomplete. The domain of unknown function (DUF) 4176 protein family is found within certain Lap and LXG encoding operons. DUF4176s stand out from the Laps due to their predicted globular shape but are similarly required for the secretion of their associated LXG effector. Interestingly, globular chaperones have previously been described for Mycobacterial T7SSa systems, suggesting that they may be an overlooked feature required for the export of T7SSb effectors as well. In this work, I demonstrate that DUF4176 genes commonly co-occur with LXG and other T7SSb-associated proteins and present the first crystal structures of two DUF4176 proteins from Streptococcus intermedius. I identify surface exposed features on these structures that may be involved in protein-protein interactions and explore possible models for how DUF4176 facilitate the secretion of LXG toxins through the T7SSb.