IS E37, A MAJOR POLYPEPTIDE OF THE INNER MEMBRANE FROM PLASTID ENVELOPE, AN S-ADENOSYL METHIONINE-DEPENDENT METHYLTRANSFERASE

Using antibodies raised against E37, one of the major polypeptides of the inner membrane from the chloroplast envelope, it has been demonstr ated that a single immunologically related polypeptide was present in total protein extracts from various higher plants (monocots and dicots ), in photosynthetic and non-photosynthetic tissues from young spinach plantlets, as well as in the cytoplasmic membrane from the cyanobacte ria Synechococcus. This ubiquitous distribution of E37 strongly sugges ts that this protein plays an envelope-specific function common to all types of plastids. Comparison of tobacco and spinach E37 amino acid s equences deduced from the corresponding cDNA demonstrates that consens us motifs for S-adenosyl methionine-dependent methyltransferases are l ocated in both sequences. This hypothesis was confirmed using a bioche mical approach. It was demonstrated that E37, together with two minor spinach chloroplast envelope polypeptides of 32 and 39 kDa, can be spe cifically photolabeled with [H-3]-S-adenosyl methionine upon UV-irradi ation. Identification of E37 as a photolabeled polypeptide was establi shed by immunoprecipitation. Furthermore, photolabeling of the three e nvelope polypeptides was specifically inhibited by very low concentrat ion of S-adenosyl homocysteine, thus providing evidence for the presen ce within these proteins of S-adenosyl methionine- and S-adenosyl homo cysteine-binding sites that were closely associated, Taken as a whole these results strongly suggest that E37 is an ubiquitous plastid envel ope protein that probably has an S-adenosyl methionine-dependent methy ltransferase activity. The 32 and 39 kDa envelope polypeptides probabl y have a similar methyltransferase activity.