Interaction of homologues of Hsp70 and Cpn60 with ferredoxin-NADP+ reductase upon its import into chloroplasts.

A homologue of the 70-kDa heat-shock protein (Hsp70) was purified from pumpkin chloroplasts. The molecular mass of the purified protein was approximately 75 kDa and its N-terminal amino acid sequence was very similar to those of homologues of Hsp70 from bacterial cells and from the mitochondrial matrix and stroma of pea chloroplasts. The purified homologue of Hsp70 was found in the stroma of chloroplasts. To investigate the role(s) of the homologue of Hsp70 in the chloroplast stroma, we examined the possibility that the homologue of Hsp70 might interact with newly imported proteins to assist in their maturation (for example, in their folding and assembly). Ferredoxin NADP+ reductase (FNR) imported into chloroplasts in vitro could be immunoprecipitated with antisera raised against the homologue of Hsp70 from pumpkin chloroplasts and against GroEL from Escherichia coli, which is a bacterial homologue of chaperonin 60 (Cpn60), in an ATP-dependent manner, an indication that newly imported FNR interacts physically with homologues of Hsp70 and Cpn60 in chloroplasts. Time-course analysis of the import of FNR showed that imported FNR interacts transiently with the homologue of Hsp70 and that the association of FNR with the homologue of Hsp70 precedes that with the homologue of Cpn60. These results suggest that homologues of Hsp70 and Cpn60 in chloroplasts might sequentially assist in the maturation of newly imported FNR in an ATP-dependent manner.