REFINED STRUCTURE OF THE PORIN FROM RHODOPSEUDOMONAS-BLASTICA - COMPARISON WITH THE PORIN FROM RHODOBACTER-CAPSULATUS

The structure of the membrane channel porin from the phototrophic bact eria Rhodopseudomonas blastica has been refined at 1.96 Angstrom resol ution yielding an R- factor of 17.6%. The final model consists of all 289 amino acid residues, 247 water molecules and three detergent molec ules modelled as n-octyltetraoxyethylene. One of these detergent molec ules binds together with its two symmetry-related molecules tightly in a pocket at the molecular 3-fold axis. This pocket may bind three alk yl chains of a lipopolysaccharide which in turn would stabilize the tr imer and could possibly play a role in membrane insertion. The overall shape of this porin resembles OmpF of Escherichia coli more than the only known sequence-related porin from Rhodobacter capsulatus. The mem brane contacting surface is similar in all structurally known porins; it shows exceptional frequencies of amino acid residues and side-chain rotamers. The 46-residue loop beta 5-beta 6 of the porin is shown to be tightly fastened to the beta-barrel, excluding an in vivo loop move ment that closes the pore. The trimer interface region has the structu re of a water-soluble protein with an extensive non-polar core and num erous hydrogen bonds at the surface. The loops at the external end of the barrel are long and rigid whereas those at the periplasmic barrel end are short and mobile. The crystal packing is discussed.