A structural foundation for studying chlamydial polymorphic membrane proteins.

Polymorphic membrane proteins (Pmps) are chlamydial cell surface proteins with signature repeating FxxN and GGA (I, V, L) tetrapeptide motifs. These autotransporter proteins are key immune targets and mediators of host-microbe interactions but by unknown mechanisms. AlphaFold predictions reveal that Pmp passenger domains have two distinct β-helical parts: a C-terminal globular "Pmp middle region" that forms six β-helical rungs, and an N-terminal highly-regular "Pmp repeat region" that is from 9 to 25 rungs long. In the repeat region, paired FxxN and GGA (I, V, L) motifs - or close variants - are fully buried and packed together to form novel β-helical rungs from which variable-length loops extend out from one face. Similar motifs in proteins from diverse organisms are predicted to form analogous structures. In Pmps, the structures imply that the tetrapeptide motifs are not directly involved in host-cell adhesion, but position protein loops on a common face where they can interact with ligands important for infection. These structural descriptions provide a guide for mutagenesis studies to identify those parts of Pmps involved in infection.IMPORTANCEInfections by bacteria in the genus Chlamydia cause a range of widespread and potentially debilitating conditions in humans and other animals. We analyzed predicted structures of a family of proteins that are potential vaccine targets found in all Chlamydia spp. Our findings deepen the understanding of protein structure, provide a descriptive framework for discussion of the protein structure, and outline regions of the proteins that may be key targets in host-microbe interactions and anti-chlamydial immunity.