A constituent of the chloroplast import complex represents a new type of GTP-binding protein.
The 34 kDa polypeptide of the outer envelope membranes from pea chloroplasts (OEP 34) is a major constituent of this membrane. OEP 34 is detected on polyacrylamide gels under non-reducing condition in association with OEP 75, the putative protein translocation pore. An antiserum against OEP 34 is able to co-immunoprecipitate the precursor of Rubisco small subunit from a partially purified import complex of chloroplast outer envelope membranes. A full-length cDNA clone coding for pea OEP 34 has been isolated. Analysis of the deduced amino acid sequence revealed typical and conserved sequence motifs found in GTP-binding proteins, making it a new and unique member of this superfamily. OEP 34 behaves as an integral constituent of the outer chloroplast envelope, which is anchored by its C-terminus into the membrane, while the majority of the protein projects into the cytoplasm. OEP 34 does not possess a cleavable N-terminal transit sequence but it is targeted to the chloroplasts and integrated into the outer membranes by internal sequence information which seems to be present in the C-terminal membrane anchor region. Productive integration of OEP 34 into the outer envelope requires, in contrast to other OEPs, protease-sensitive chloroplast surface components and is stimulated by ATP. The GTP binding specificity of OEP 34 is demonstrated by photo-affinity labelling in the presence of [alpha-32P]GTP. Overexpressed and purified OEP 34 possesses endogenous GTPase activity. These results indicate a possible regulatory function of OEP 34 in protein translocation into chloroplasts.
Seedorf
M
M
Botanisches Institut, Universität Kiel, Germany.
Waegemann
K
K
Soll
J
J
D000595:Amino Acid Sequence; D001483:Base Sequence; D001693:Biological Transport, Active; D002736:Chloroplasts / Q000378:metabolism; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D018532:Pisum sativum / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
D018076:DNA, Complementary; D018744:DNA, Plant; D010940:Plant Proteins; C093715:outer envelope membrane protein 34-kDa; D019204:GTP-Binding Proteins