A monomeric, tightly folded stromal intermediate on the delta pH-dependent thylakoidal protein transport pathway.
Two distinct mechanisms have been previously identified for the transport of proteins across the chloroplast thylakoid membrane, one of which is unusual in that neither soluble factors nor ATP are required; the system requires only the transthylakoidal delta pH. We have examined this mechanism by studying the properties of one of its substrates: the extrinsic 23-kDa protein (23K) of photosystem II. Previous work has shown that this protein can be transported into isolated thylakoids as the full-length precursor protein; we show that the stromal import intermediate form of this protein is similarly translocation-competent. Gel filtration tests indicate that the stromal intermediate is probably monomeric. Protease sensitivity tests on both the initial in vitro translation product and the stromal import intermediate show that the presequence is highly susceptible to digestion whereas the mature protein is resistant to high concentrations of trypsin. The mature protein becomes very sensitive to digestion if unfolded in urea, or after heating, and we therefore propose that the natural substrate for this translocation system consists of a relatively unfolded presequence together with a tightly folded passenger protein. The ability of thylakoids to import pre-23K is destroyed by prior treatment of the thylakoids with low concentrations of trypsin, demonstrating the involvement of surface-exposed proteins in the import process. However, we can find no evidence for the binding of pre-23K or i23K to the thylakoid surface, and we therefore propose that the initial interaction of these substrates with the thylakoidal translocase is weak, reversible, and probably delta pH-independent. In the second phase of the translocation mechanism, the delta pH drives either the translocation and unfolding of proteins, or the translocation of a fully folded protein.