The crystallographic structure of Neisseria gonorrhoeae pilin, which assembles into the multifunctional pilus adhesion and virulence factor, reveals an alpha-beta roll fold with a striking 85 Angstrom alpha-helical spine and an O-linked disaccharide. Hey residues stabilize interactions that allow sequence hypervariability, responsible for pilin’s celebrated antigenic variation, within disulphide region beta-strands and connections. Pilin surface shape, hydrophobicity and sequence variation constrain pilus assembly to the packing of flat subunit faces against alpha(1) helices. Helical fibre assembly is postulated to form a core of coiled alpha(1) helices banded by beta-sheet, leaving carbohydrate and hypervariable sequence regions exposed to solvent.