Pili--filamentous protein structures found on the cell surface of many infectious gram-negative bacteria--often are virulence factors that mediate adherence to host epithelial cells. The pilus, a major surface antigen of the gonococcus, is formed by the specific association of thousands of repeating identical protein subunits (pilin). Structural studies of the pilin protein and the pilus fiber may aid the rational design of a peptide-based vaccine by providing information on the antigenic surface of pili that includes the identification of sequence-distant antigenic regions that are localized to single areas of the three-dimensional structure. Preliminary results suggest that the pilin subunit has structural similarity to the 4-alpha-helix bundle fold in proteins such as the coat protein subunit of tobacco mosaic virus and myohemerythrin. Therefore, the monomeric protein myohemeythrin was used to identify structural correlations for antigenic determinants on 4-alpha-helix bundle proteins such as pilin.