STRUCTURE OF THE MEMBRANE CHANNEL PORIN FROM RHODOPSEUDOMONAS-BLASTICA AT 2.0 ANGSTROM RESOLUTION

The crystal structure of a membrane channel, homotrimeric porin from R hodopseudomonas blastica has been determined at 2.0 Angstrom resolutio n by multiple isomorphous replacement and structural refinement. The c urrent model has an R-factor of 16.5% and consists of 289 amino acids, 238 water molecules, and 3 detergent molecules per subunit. The parti al protein sequence and subsequently the complete DNA sequence were de termined. The general architecture is similar to those of the structur ally known porins. As a particular feature there are 3 adjacent bindin g sites for n-alkyl chains at the molecular 3-fold axis. The side chai n arrangement in the channel indicates a transverse electric field acr oss each of the 3 pore eyelets, which may explain the discrimination a gainst nonpolar solutes. Moreover, there are 2 significantly ordered g irdles of aromatic residues at the nonpolar/polar borderlines of the i nterface between protein and membrane. Possibly, these residues shield the polypeptide conformation against adverse membrane fluctuations.