N-Terminal Sequences of Subunits L and M of the Photosynthetic Reaction Centre fromRhodospirillum rubrumG-9+. Separation of the Subunits by Gel Filtration on Hydroxypropylated Sephadex G 100 in Organic Solvents

Abstract

A new method was developed by which subunits L and M of the photosynthetic reaction center from R. rubrum G-9+ can be obtained in pure form, starting from freeze-dried chromatophore membranes. The method employs extraction into a mixture of chloroform/methanol and gel permeation chromatography on a column of hydroxypropylated Sephadex G 100. Cross-contamination of the purified subunits was < 5% (mol/mol), as estimated by manual Edman degradation. Automated Edman degradation was carried out with both subunits in a liquid-phase sequencer; 36 amino-acid residues of subunit L and 50 residues of subunit M could be unequivocally identified. In both cases, the sequence analyses came to a premature end as the signal dropped to the level of the accidental fluctuations of the phenylthiohydantoin-derivatives background. This effect is explained by the unusual susceptibility to peptide bond cleavage of certain threonine residues which probably underwent N .fwdarw. O acyl shift during the cleavage reactions. The N-terminal sequences were compared to those of subunits L and M of the photosynthetic reaction center from Rhodopseudomonas sphaeroides R-26. The homology among subunits L is .apprx. 90% and thus markedly higher than that among subunits M (32%). This finding indicates a pre-eminent role of subunit L in the primary events of photosynthetic energy conversion.