Abstract
ABSTRACTSophisticated nanomachines are used by bacteria for protein secretion. In Gram-negative bacteria, the type 2 secretion system (T2SS) is composed of a pseudopilus assembly platform in the inner membrane and a secretin complex in the outer membrane. The engagement of these two megadalton-sized complexes is required in order to secrete toxins, effectors, and hydrolytic enzymes. Pseudomonas aeruginosa has at least two T2SSs, with the ancestral nanomachine having a secretin complex composed of XcpQ. Until now, no high-resolution structural information was available to distinguish the features of this Pseudomonas-type secretin, which varies greatly in sequence from the well-characterized Klebsiella-type and Vibrio-type secretins. We have purified the ~1-MDa secretin complex and analyzed it by cryo-electron microscopy. Structural comparisons with the Klebsiella-type secretin complex revealed a striking structural homology despite the differences in their sequence characteristics. At 3.6-Å resolution, the secretin complex was found to have 15-fold symmetry throughout the membrane-embedded region and through most of the domains in the periplasm. However, the N1 domain and N0 domain were not well ordered into this 15-fold symmetry. We suggest a model wherein this disordering of the subunit symmetry for the periplasmic N domains provides a means to engage with the 6-fold symmetry in the inner membrane platform, with a metastable engagement that can be disrupted by substrate proteins binding to the region between XcpP, in the assembly platform, and the XcpQ secretin.
Highlights
Sophisticated nanomachines are used by bacteria for protein secretion
® mbio.asm.org 1 inner membrane protein complex that functions as an assembly platform and ATPdependent motor, and from this platform, the pili are driven into a cavernous secretin complex embedded in the outer membrane that spans the periplasm to engage the inner membrane
The pseudopilus is driven into the type 2 secretion system (T2SS) secretin complex by either a piston or a screw-like mechanism, thereby promoting secretion of substrate proteins across the outer membrane
Summary
Sophisticated nanomachines are used by bacteria for protein secretion. In Gram-negative bacteria, the type 2 secretion system (T2SS) is composed of a pseudopilus assembly platform in the inner membrane and a secretin complex in the outer membrane. The pseudopilus is driven into the T2SS secretin complex by either a piston or a screw-like mechanism, thereby promoting secretion of substrate proteins (toxins, effectors, and hydrolytic enzymes) across the outer membrane. Recent highresolution structures of the Vibrio-type and Klebsiella-type secretins showed them to be of 15-fold symmetry [4].
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