Stromal factor plays an essential role in protein integration into thylakoids that cannot be replaced by unfolding or by heat shock protein Hsp70.

The light-harvesting chlorophyll a/b protein (LHCP) is an integral thylakoid membrane protein. It is made in the cytosol as a precursor (pLHCP), imported into chloroplasts, and subsequently integrated into thylakoids. Integration of pLHCP into thylakoids requires a stromal protein factor that functions in part to maintain the solubility and integration competence of pLHCP. Recently, it was reported that unfolded pLHCP was sufficient for integration and that the stromal factor, identified as the plastid Hsp70, was required only to prevent pLHCP refolding [Yalovsky, S., Paulsen, H., Michaeli, D., Chitnis, P. R. & Nechushtai, R. (1992) Proc. Natl. Acad. Sci. USA 89, 5616-5619]. Our studies, using more rigorous criteria for integration, show that unfolded pLHCP is not sufficient; stromal factor is an absolute requirement for integration. Furthermore, experiments with purified Hsp70 as well as Hsp70-depleted stromal extract demonstrate that Hsp70 is not the stromal factor. These results plus the finding that pLHCP diluted out of urea is relatively stable as a substrate for integration point to an additional role for the stromal factor in targeting and/or membrane translocation.