A chloroplast processing enzyme involved in precursor maturation shares a zinc-binding motif with a recently recognized family of metalloendopeptidases.
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 beta 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.
VanderVere
P S
PS
Department of Molecular Genetics and Cell Biology, University of Chicago, IL 60637, USA.
Bennett
T M
TM
Oblong
J E
JE
Lamppa
G K
GK
D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites; D002736:Chloroplasts / Q000201:enzymology*; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D008666:Metalloendopeptidases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D015394:Molecular Structure; D008970:Molecular Weight; D018532:Pisum sativum / Q000201:enzymology / Q000235:genetics; D010940:Plant Proteins / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000378:metabolism
D018076:DNA, Complementary; D018744:DNA, Plant; D010940:Plant Proteins; D011498:Protein Precursors; D008666:Metalloendopeptidases; C046583:pitrilysin; D015032:Zinc