The major outer membrane protein (MOMP) of Campylobacter jejuni was purified to homogeneity by selective solubilization and fast protein liquid chromatography. The amino acid composition of the MOMP indicates the presence of cysteine residues. The amino-terminal sequence, determined over 31 residues, shows no significant homology with any other porin from gram-negative bacteria except in a discrete region. Immunocross-reactivity between Escherichia coli OmpC and the MOMP was analyzed, and a common antigenic site between these two porins was identified with an anti-peptide antibody. From circular dichroism and immunological investigations, the existence of a stable folded monomer, containing a high level of ␤-sheet secondary structure, is evident. Conformational analyses show the presence of a native trimeric state generated by association of the three folded monomers; the stability of this trimer is reduced compared with that of E. coli porins. This study clearly reveals that the C. jejuni MOMP is related to the family of trimeric bacterial porins.The gram-negative bacterium Campylobacter jejuni is responsible for enteritis and diarrhea all over the world, in both industrialized and developing countries (41). Since 1970, this bacterium and other Campylobacter species have been routinely isolated from stools and other specimens. Numerous studies that have tried to elucidate the molecular mechanisms of pathogenicity have been done in several laboratories. Flagella play a role in mobility in the luminal mucosa, which appears to be one of the most important steps in the colonization of the host gut (14,15,33). Moreover, little is known about the structural and functional organization of the bacterial envelope.Porins of gram-negative bacteria are involved in the diffusion of solutes through this envelope and are the most common way of communication between bacteria and media (16,34). In Escherichia coli, the major outer membrane porins, OmpF and OmpC, have different pore sizes and their synthesis depends on external conditions (36). A previous report has established that the major outer membrane protein (MOMP) of C. jejuni presents the physicochemical properties of a porin (18). Although the stabilities of porin trimers are a peculiarity of these membrane pore proteins (32, 38), there are some divergent views on the quaternary structure of this MOMP (1,18,27). Recent studies of E. coli OmpA have reported a pore activity for this monomeric protein previously described as an architectural component of the bacterial envelope (36, 40). Similarly, a monomeric porin, showing noticeable homology with the OmpA sequence, was also isolated from Pseudomonas aeroginosa (2, 7). In addition, the electrophoretic migrations of these proteins are similarly modified by the temperature of solubilization (6,7,18). These results suggest that monomeric OmpA-like proteins form channels in the outer membranes of gram-negative bacteria (35).The purification and determination of the amino acid composition and amino-terminal sequence of the MO...