Analysis of the PilQ Secretin from Neisseria meningitidis by Transmission Electron Microscopy Reveals a Dodecameric Quaternary Structure

Abstract

PilQ is a member of the secretin family of outer membrane proteins and is specifically involved in secretion of type IV pili in Neisseria meningitidis, Neisseria gonorrhoeae , and Pseudomonas aeruginosa . The quaternary structure of PilQ from N. meningitidis was analyzed by transmission electron microscopy by using a negative stain. Single particle averaging was carried out with a total data set of 650 individual particles, which produced a projection 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 diameter. Self-rotation and 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, based on the X-ray crystal structure of the N. gonorrhoeae pilin subunit, fitted neatly into the cavity, demonstrating how PilQ could serve as a channel for the growing pilus fiber.