Analysis of the PilQ secretin from Neisseria meningitidis by transmission electron microscopy reveals a dodecameric quaternary structure

PilQ is a member of the secretin family of outer membrane proteins and is s pecifically involved in secretion of type TV pill in Neisseria meningitidis , Neisseria gonorrhoeae, and Pseudomonas aeruginosa, The quaternary structu re of PilQ from N. meningitidis was analyzed by transmission electron micro scopy by using a negative stain. Single particle averaging was carried out with a total data set of 650 individual particles, which produced a project ion map generated from 296 particles at an estimated resolution of 2.6 nm, Oligomeric PilQ adopts a donut-like structure with an external ring that is 16.5 nm in diameter surrounding a central cavity that is 6.5 nm in diamete r. Self-rotation acid power spectrum analysis demonstrated the presence of 12-fold rotational symmetry, showing that PilQ is organized as a ring of 12 identical subunits, A model of the type IV meningococcal pilus fiber, base d on the X-ray crystal structure of the N. gonorrhoeae pilin subunit, fitte d neatly into the cavity, demonstrating how PilQ could serve as a channel f or the growing pilus fiber.