Smith T A, Kohorn B D
Botany Department, Duke University, Durham, North Carolina 27708.
J Cell Biol. 1994 Jul;126(2):365-74. doi: 10.1083/jcb.126.2.365.
The apparatus that permits protein translocation across the internal thylakoid membranes of chloroplasts is completely unknown, even though these membranes have been the subject of extensive biochemical analysis. We have used a genetic approach to characterize the translocation of Chlamydomonas cytochrome f, a chloroplast-encoded protein that spans the thylakoid once. Mutations in the hydrophobic core of the cytochrome f signal sequence inhibit the accumulation of cytochrome f, lead to an accumulation of precursor, and impair the ability of Chlamydomonas cells to grow photosynthetically. One hydrophobic core mutant also reduces the accumulation of other thylakoid membrane proteins, but not those that translocate completely across the membrane. These results suggest that the signal sequence of cytochrome f is required and is involved in one of multiple insertion pathways. Suppressors of two signal peptide mutations describe at least two nuclear genes whose products likely describe the translocation apparatus, and selected second-site chloroplast suppressors further define regions of the cytochrome f signal peptide.
尽管叶绿体内部类囊体膜已经成为广泛生化分析的对象，但允许蛋白质跨叶绿体内部类囊体膜转运的机制却完全未知。我们采用遗传学方法来表征衣藻细胞色素f的转运情况，细胞色素f是一种叶绿体编码的蛋白质，它单次跨类囊体膜。细胞色素f信号序列疏水核心区的突变会抑制细胞色素f的积累，导致前体的积累，并损害衣藻细胞光合生长的能力。一个疏水核心突变体还会减少其他类囊体膜蛋白的积累，但不会减少那些完全跨膜转运的蛋白的积累。这些结果表明，细胞色素f的信号序列是必需的，并且参与了多种插入途径中的一种。两个信号肽突变的抑制子描述了至少两个核基因，其产物可能描述了转运机制，而选定的第二位点叶绿体抑制子进一步定义了细胞色素f信号肽的区域。
