Structural similarities between the major polypeptides of thylakoid membranes from Chlamydomonas reinhardtii

The major polypeptides of thylakoid membranes from Chlamydomonas reinhardtii were purified by preparative gel electrophoresis and examined for structural similarities. The largest of these polypeptides has an apparent molecular mass of 29,500 ± 500 daltons, whereas the other two both have an apparent mass of 26,000 ± 500 daltons. The amino acid compositions and uv-absorption spectra of the 29K- and 26K-dalton polypeptides are very similar. The same pattern of release of amino acids was obtained from both fractions by digestion with carboxypeptidase Y. Endoproteolytic digestion with trypsin, chymotrypsin, staphylococcal protease, and mild acid yielded identical patterns of N-terminal amino acids from both the 29K- and 26K-dalton polypeptides. However, different patterns of peptides were found after electrophoresis of fragments generated by digestion with staphylococcal protease. Conditions of electrophoresis were defined that permitted separation of the 26K-dalton fraction into two components, designated as polypeptides 16 and 17 in the identification system of Chua and Bennoun (1975, Proc. Nat. Acad. Sci. USA72, 2175–2179). Amino acid compositions of these two polypeptides are nearly identical. Polypeptide 16 contained N-terminal isoleucine, but no free N-terminal amino group was detected in polypeptide 17. Electrophoretic analysis of staphylococcal protease digests of these two polypeptides revealed significant differences in the patterns of peptides. These data confirm that there are three distinct major polypeptides in these membranes, which are present at nearly equal amounts. However, the data also suggest that significant similarities in amino acid sequence exist between these polypeptides.