During assembly of the bacterial flagellum, the proteins that form the exterior structures are exported through the growing structure by an export apparatus at the base that is both very fast (translocating several large protein molecules per second) and very specific (acting on just a few cargo proteins from among the hundreds of proteins in the cell). A related process occurs in many pathogenic bacteria to export virulence factors that compromise host-cell defenses. This project seeks to understand the molecular basis of cargo recognition, using NMR methods to examine the structures and interactions of purified cargo and export-apparatus proteins.