The porin of FR1 was found in the outer membrane as a complex with a relatively small (32 kDa) porin-associated protein (PAP). The porin moiety of the complex consisted of a trimer which revealed a mainly β-sheet structure, while the porin-PAP complex also contained a significant α-helical portion. Isolated PAP exhibited a mostly α-helical structure. The porin-PAP complex had the same single-channel conductivity (2.8 nS) as the isolated porin, demonstrating that PAP did not affect the porin channel size. Differential extraction of the cell wall fraction with N,N-dimethyldodecylamine N-oxide revealed that PAP stabilized the porin in the outer membrane. EDTA caused dissociation of the porin-PAP complex, indicating that divalent cations were involved in formation of this complex.