Expression, Purification, and Cryo-EM Structural Analysis of an Outer Membrane Secretin Channel

Secretin proteins form pores in the outer membranes of Gram-negative bacteria, and as such provide a means of transporting a wide variety of molecules out of or in to the cell. They are important components of several different bacterial secretion systems, surface filament assembly machineries, and virus assembly complexes. Despite accommodating a diverse assortment of molecules, including virulence factors, folded proteins, and whole viruses, the secretin family of proteins is highly conserved, particularly in their membrane-embedded β-barrel domain. We describe here a protocol for the expression, purification and cryo-EM structural determination of the pIV secretin from the Ff family of filamentous bacteriophages.