Periplasmic coiled coil formed by assembly platform proteins PulL and PulM is critical for function of the Klebsiella type II secretion system

Bacteria use type II secretion systems (T2SS) to secrete to their surface folded proteins that confer diverse functions, from nutrient acquisition to virulence. In the Klebsiella species, T2SS-mediated secretion of pullulanase (PulA) requires assembly of a dynamic filament called pseudopilus. The inner membrane assembly platform (AP) complex is essential for PulA secretion and pseudopilus assembly. The AP components PulL and PulM form an inner membrane complex interacting through their C-terminal globular domains and transmembrane segments. Here we investigated the roles of periplasmic helices and cytoplasmic domains of PulL and PulM in their assembly. We found that PulL and PulM variants lacking periplasmic helices were defective for interactions in the bacterial two-hybrid (BACTH) assay. Their function in PulA secretion and assembly of PulG subunits into pseudopilus filaments were strongly reduced. In addition, deleting the cytoplasmic peptide of PulM in variant PulMΔN nearly abolished interaction with PulG in the BACTH assay, without affecting the interaction with PulL. Nevertheless, PulL was degraded in the presence of the PulMΔN variant, suggesting that PulM N-terminal peptide interacts with PulL in the cytoplasm and plays a stabilizing role. We discuss the implication of these results for the mechanism of T2S and type IV pilus assembly. ### Competing Interest Statement The authors have declared no competing interest.