THE MAJOR OUTER-MEMBRANE PROTEIN OF CHLAMYDIA-PSITTACI FUNCTIONS AS APORIN-LIKE ION-CHANNEL

The major outer membrane protein (MOMP) of Chlamydia species shares se veral biochemical properties with classical porin proteins. Secondary structure analysis by circular dichroism now reveals that MOMP purifie d from Chlamydia psittaci has a predominantly beta-sheet content (62%) , which is also typical of bacterial porins. Can MOMP form functional ion channels? To directly test the ''porin channel'' hypothesis at the molecular level, the MOMP was reconstituted into planar lipid bilayer s, where it gave rise to multibarreled channels, probably trimers, whi ch were modified by an anti-MOMP monoclonal antibody. These observatio ns are consistent with the well-characterized homo-oligomeric nature o f MOMP previously revealed by biochemical analysis and with the triple -barreled behavior of other porins. MOMP channels were weakly anion se lective (P-Cl/P-K similar to 2) and permeable to ATP. They may therefo re be a route by which Chlamydia can take advantage of host nucleoside triphosphates and explain why some anti-MOMP antibodies neutralize in fection. These findings have broad implications on the search for an e ffective chlamydial vaccine to control the significant human and anima l diseases caused by these organisms.