Structural insights into the secretin translocation channel in the type II secretion system

The secretin GspD of the type II secretion system (T2SS) forms a channel across the outer membrane in Gram-negative bacteria to transport substrates from the periplasm to the extracellular milieu. The lack of an atomic-resolution structure of the GspD channel hinders the investigation of substrate translocation mechanism of T2SS. Here we report cryo-EM structures of two GspD channels (similar to 1 MDa), from Escherichia coli K12 and Vibrio cholerae, at similar to 3 angstrom resolution. The structures reveal a pentadecameric channel architecture, wherein three rings of GspD N domains form the periplasmic channel. The secretin domain constitutes a novel double beta-barrel channel, with at least 60 beta-strands in each barrel, and is stabilized by S domains. The outer membrane channel is sealed by beta-strand-enriched gates. On the basis of the partially open state captured, we proposed a detailed gate-opening mechanism. Our structures provide a structural basis for understanding the secretin superfamily and the mechanism of substrate translocation in T2SS.