Sec-dependent thylakoid protein translocation. Delta pH requirement is dictated by passenger protein and ATP concentration.

A Sec-type system is responsible for the translocation of a subset of proteins across the thylakoid membrane in higher plant chloroplasts. Previous studies have suggested that the thylakoidal delta pH plays a minor role in this translocation mechanism, but we show here that it can be essential for the translocation process, depending on the identity of the passenger protein and the concentration of ATP. Studies using chimeric proteins show that, whereas the presequence dictates the translocation pathway, the delta pH requirement is dictated exclusively by the passenger protein; some passenger proteins are virtually delta pH-independent whereas others are absolutely dependent. delta pH requirement is not related to charge characteristics of the passenger proteins, ruling out an electrophoretic effect. Analysis of the 33-kDa photosystem II protein reveals an inverse relationship between delta pH requirement and ATP concentration; import into isolated thylakoids is inhibited 14-fold by nigericin at moderate ATP concentrations, and totally inhibited when the ATP concentration is reduced to 2 microM. The results indicate that the roles of the delta pH and ATP overlap and suggest that the delta pH may be obligatory when the passenger protein is abnormally difficult to translocate, possibly due to the folding of the polypeptide chain. We compare the energetics of this system with those of prokaryotic systems from which the chloroplast system is believed to have evolved.