Porin of Rhodobacter capsulatus: biochemical and functional characterization.

The major outer membrane protein of Rhodobacter capsulatus 37b4 (capsule-free) was isolated. Strong porin-activity was observed after reconstitution into artificial lipid bilayer membranes with a single channel conductance of 3.15 nS in 1 M KCl. The porin migrated as a broad, single band (Mr above 90,000) on sodium dodecyl sulfate polyacrylamide gel electrophoresis and dissociated into a single species of polypeptides (Mr 36,000) on treatment with EDTA (10 mM at 30 degrees C, 20 min) or by heating (100 degrees C, 5 min). Analytical ultracentrifugation studies demonstrated the native porin to be a trimer. The monomers chromatofocused as a single, sharp peak on fast performance liquid chromatography and only one band, corresponding to an isoelectric point of about 4.0, was obtained on isoelectric focusing. Gas-phase sequencing of the 23 N-terminal residues revealed Glu-Val-Lys-Leu-Ser-Gly-Asp-Ala-Arg-Met-Gly-Val-Met-Tyr-Asn-Gly-Asp-Asp- X-Asn- Phe-Ser-Ser.