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A Chloroplast Processing Enzyme Involved in Precursor Maturation Shares a Zinc-Binding Motif with a Recently Recognized Family of Metalloendopeptidases
Pamela S. Vander Vere, Thomas M. Bennett, John E. Oblong and Gayle K. Lamppa
Proceedings of the National Academy of Sciences of the United States of America
Vol. 92, No. 16 (Aug. 1, 1995), pp. 7177-7181
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/2368207
Page Count: 5
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Nuclear-encoded proteins targeted to the chloroplast are typically synthesized with N-terminal transit peptides which are proteolytically removed upon import. Structurally related proteins of 145 and 143 kDa copurify with a soluble chloroplast processing enzyme (CPE) that cleaves the precursor for the major light-harvesting chlorophyll a/b binding protein and have been implicated in the maturation of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and acyl carrier protein. The 145- and 143-kDa proteins have not been found as a heterodimer and thus may represent functionally independent isoforms encoded by separate genes. Here we describe the primary structure of a 140-kDa polypeptide encoded by cDNAs isolated by using antibodies raised against the 145/143-kDa doublet. The 140-kDa polypeptide contains a transit peptide, and strikingly, a His-Xaa-Xaa-Glu-His zinc-binding motif that is conserved in a recently recognized family of metalloendopeptidases, which includes Escherichia coli protease III, insulin-degrading enzyme, and subunit β of the mitochondrial processing peptidase. Identity of 25-30%, concentrated near the N terminus of the 140-kDa polypeptide, is found with these proteases. Expression of CPE in leaves is not light dependent. Indeed, transcripts are present in dark-grown plants, and the 145/143-kDa doublet and proteolytic activity are both found in etioplasts, as well as in root plastids. Thus, CPE appears to be a necessary component of the import machinery in photosynthetic and nonphotosynthetic tissues, and it may function as a general stromal processing peptidase in plastids.
Proceedings of the National Academy of Sciences of the United States of America © 1995 National Academy of Sciences