A STRONG PROTEIN UNFOLDING ACTIVITY IS ASSOCIATED WITH THE BINDING OFPRECURSOR CHLOROPLAST PROTEINS TO CHLOROPLAST ENVELOPES

Protein conformational changes related to transport into chloroplasts have been studied. Two chimaeric proteins carrying the transit peptide of either ferredoxin or plastocyanin linked to the mouse cytosolic en zyme dihydrofolate reductase (EC 1.5.1.3.) were employed. In contrast to observations in mitochondria, we found in chloroplasts that transpo rt of a purified ferredoxin-dihydrofolate reductase fusion protein is not blocked by the presence of methotrexate, a folate analogue that st abilizes the structural conformation of dihydrofolate reductase. It is shown that transport competence of this protein in the presence of me thotrexate is not a consequence of alteration of the folding character istics or methotrexate binding properties of dihydrofolate reductase b y fusion to the ferredoxin transit peptide. Binding of dihydrofolate r eductase fusion proteins to chloroplast envelopes is not inhibited by low temperature and it is only partially diminished by methotrexate. I t is demonstrated that the dihydrofolate reductase fusion proteins unf old, despite the presence of methotrexate, on binding to the chloropla st envelopes. We propose the existence of a strong protein unfolding a ctivity associated to the chloroplast envelopes.