Yuan J, Henry R, Cline K
Horticultural Sciences Department, University of Florida, Gainesville 32611.
Proc Natl Acad Sci U S A. 1993 Sep 15;90(18):8552-6. doi: 10.1073/pnas.90.18.8552.
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.
捕光叶绿素a/b蛋白（LHCP）是一种内在的类囊体膜蛋白。它在细胞质中以前体形式（pLHCP）合成，被导入叶绿体，随后整合到类囊体中。pLHCP整合到类囊体中需要一种基质蛋白因子，该因子部分作用是维持pLHCP的溶解性和整合能力。最近有报道称，未折叠的pLHCP足以进行整合，而被鉴定为质体Hsp70的基质因子仅用于防止pLHCP重新折叠[Yalovsky, S., Paulsen, H., Michaeli, D., Chitnis, P. R. & Nechushtai, R. (1992) Proc. Natl. Acad. Sci. USA 89, 5616 - 5619]。我们使用更严格的整合标准进行的研究表明，未折叠的pLHCP并不足够；基质因子是整合的绝对必要条件。此外，用纯化的Hsp70以及耗尽Hsp70的基质提取物进行的实验表明，Hsp70不是基质因子。这些结果加上从尿素中稀释出来的pLHCP作为整合底物相对稳定这一发现，表明基质因子在靶向和/或膜转运中还有额外作用。
