Structure and dynamic association of an assembly platform subcomplex of the bacterial type II secretion system

Abstract

Type II secretion systems (T2SS) allow diderm bacteria to secrete hydrolytic enzymes, adhesins or toxins important for growth and virulence. In T2SS, secretion of folded proteins from the periplasm to the cell surface requires assembly of periplasmic filaments called pseudopili. Like the related type IV pili, pseudopili are polymerized in the inner membrane through addition of subunits at the filament base, mediated by the essential assembly platform (AP). To understand the structure and molecular role of the AP, we focused on its components PulL and PulM from the Klebsiella oxytoca T2SS. By combining biophysical methods, NMR and X-ray crystallography we studied the structure and associations of their periplasmic domains. We describe the first structure of the heterodimer complex formed by the PulL and PulM ferredoxin-like domains and show how their structural complementarity and plasticity favor their association during the secretion process. Cysteine scanning and cross-linking of transmembrane segments provided additional constraints to build a structural model of the PulL-PulM complex and assembly in the cellular context. Together with the relative abundance of PulL, PulM and their partners our findings suggest a model of the AP as a dynamic hub that orchestrates pseudopilus polymerization.