A constituent of the chloroplast import complex represents a new type of GTP‐binding protein

Summary 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 ATR. The GTP binding specificity of OEP 34 is demonstrated by photo‐affinity labelling in the presence of [α‐ 32 P]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.