While seeing is believing, some proteins are harder to image than others because they are membrane-embedded, don’t express well, are prone to degradation, etc. The type IV pilus secretin, which is used by the pilus to exit the cell, is a massive (>1 MDa) membrane embedded complex which has been tough to image with approaches other than electron microscopy, which until recently has provided limited resolution. Jason Koo, a former PDF in the Howell lab, worked with a talented electron microscopist, John Rubinstein from Toronto’s Sick Kids Research Institute, to get a sub-nanometre structure of PilQ secretins isolated directly from P. aeruginosa. The unprecedented resolution of this structure allows us to make predictions about its stoichiometry (14 subunits in a ring) and symmetry (C7 with alternating large and small spokes on the periphery). We can also predict how the outer membrane segment may look (14 x 4 beta strands from each subunit), and see that the only conformational changes needed to fit a pilus inside would be displacement of the internal gate. In related bacteria a secretin-associated protein called TsaP was important for pilus function; although P. aeruginosa has a TsaP orthologue, Ryan Lamers, a PDF in the Burrows lab, showed that its deletion had no effect on pilus expression or function. Congratulations all on this new benchmark for secretin structural information.
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