Remodeling of the Chlamydia psittaci inclusion by the type III secreted, BAR domain-containing inclusion membrane protein IncA

Abstract

Chlamydia species cause infections in humans that range from prevalent, often asymptomatic genital infections caused by Chlamydia trachomatis to relatively rare, potentially life-threatening zoonotic infections caused by avian Chlamydia psittaci. All Chlamydia spp. encode integral membrane proteins (Incs) that are type III-secreted through the injectisome from the bacterial cell into the host-derived inclusion membrane where they interact with host proteins. These include Incs of C. trachomatis that interact with membrane curvature-inducing BAR-domain sorting nexins (SNX), potentially disrupting retromer-mediated endosome-to-Golgi trafficking to benefit the pathogen. Eukaryotic BAR domain proteins interact with bacterial proteins, but prokaryotic BAR domain proteins have yet to be confirmed. I propose that IncA/Cps, the first BAR protein described in a prokaryote, specifically contributes to the remodeling of the C. psittaci inclusion membrane to form a) IncA-laden retromer-like tubules extending outward into the cytosol, and/or b) folds and tubular extensions extending inward into the inclusion lumen. IncA of C. psittaci (IncA/Cps) contains a predicted SNX-BAR-like domain that is absent in the C. trachomatis ortholog. Comparative sequence analysis of IncA orthologs across Chlamydia spp. suggests that the SNX-BAR-like domain of IncA/Cps was acquired by convergent evolution. IncA/Cps BAR-mediated in vitro tubulation of liposomes was observed. Transfected HEK293 cells expressed filamentous/tubular structures laden with IncA/Cps. These observations suggest that IncA/Cps has membrane remodeling activity. During C. psittaci infection, IncA/Cps localized to the inclusion membrane and to nocodazole-sensitive retromer-like tubules extending from the inclusion membrane into the host cytosol. IncA-specific immunofluorescence staining of the C. psittaci inclusion displayed a characteristic irregular configuration with concave pits and folds extending into the inclusion lumen, sharply contrasting the characteristic ovoid shape of the C. trachomatis inclusion. In addition, abundant luminal IncA/Cps staining did not colocalize with growing chlamydiae and was sensitive to host sphingolipid biosynthesis inhibition. This suggests that luminal IncA/Cps is structurally continuous with the inclusion membrane. Inward extensions may enhance inclusion membrane contact-dependent growth of C. psittaci and eventually infectious progeny per infected cell. The higher infectious load may contribute to C. psittaci-induced severe pathologies and high transmission rates between birds and to humans.