THE PORINS FROM THE HALOPHILIC SPECIES ECTOTHIORHODOSPIRA-SHAPOSHNIKOVII AND ECTOTHIORHODOSPIRA-VACUOLATA

Major outer membrane proteins with porin activity were isolated from c ell envelopes of the halophilic strains Ectothiorhodospira shaposhniko vii N1 md Ectothiorhodospira vacuolata beta 1. The porins were obtaine d as oligomers. They dissociated into monomers by heat or EDTA treatme nt. The molecular masses of the monomers were determined by mass spect rometry to be 39,285 and 37,160 Da for E. shaposhnikovii N1 and E. vac uolata beta 1, respectively. Both were shown by analytical ultracentri fugation to be trimers of about 112,000 Da. Circular dichroism spectra indicated predominantly beta-sheet structure. The 18 N-terminal amino acid sequences of the two porins were identical except for the amino acids in positions 12 and 14. No sequence similarity with the primary structure of known porins was found. In reconstitution experiments wit h lipid bilayers, the porins of E. shaposhnikovii N1 and E. vacuolata beta 1 formed channels with a single-channel conductance of 1.5 and 0. 7 n, respectively, in 1 M KCl. The single-channel conductance saturate d with increasing salt concentration, indicating a putative binding-si te for anions in the channel since both porins exhibited anion-selecti vity. For the porin of E. vacuolata beta 1, but not for that of E. sha poshnikovii N1, an influence of detergent concentration on the single- channel conductance was observed.