THE MAJOR CHLOROPLAST ENVELOPE POLYPEPTIDE IS THE PHOSPHATE TRANSLOCATOR AND NOT THE PROTEIN IMPORT RECEPTOR

DURING photosynthetic CO2 fixation, fixed carbon is exported from the chloroplasts in the form of triose phosphate by the chloroplast phosphate translocator, which is the principal polypeptide (E29) from spinach chloroplast envelopes 1. We have sequenced this nuclear-coded envelope membrane protein from both spinach and pea chloroplasts 2,3. An envelope membrane protein, E30, has been identified as a possible receptor for protein import into pea chloroplasts using an anti-idiotypic antibody approach 4-6; antibodies raised against purified E30 inhibited binding and import of proteins into chloroplasts 7. The amino-acid sequence of E30 deduced from its complementary DNA 7 turned out to be highly homologous to that of E29, assigned by us as the spinach phosphate translocator 2, and was identical to the corresponding polypeptide from pea chloroplasts 3. Differences in the binding properties to hydroxylapatite of E30 and the phosphate translocator suggested that E30 was not responsible for the chloroplast phosphate-transport activity but was the chloroplast import receptor 7. Here we present evidence that argues against this and which identifies E30 as the chloroplast phosphate translocator.