Beasley E M, Müller S, Schatz G
Biocenter, University of Basel, Switzerland.
EMBO J. 1993 Jun;12(6):2303-11. doi: 10.1002/j.1460-2075.1993.tb05884.x.
Cytochrome b2, a protein of the yeast mitochondrial intermembrane space, is synthesized with an 80 residue bipartite presequence. The amino-terminal portion resembles a matrix-targeting signal. The carboxy-terminal portion acts as a 'sorting signal' for the intermembrane space and contains a hydrophobic stretch. In order to define this sorting signal, we fused the first 167 residues of the cytochrome b2 precursor to a passenger protein, expressed the fusion protein in yeast and selected for mutations that caused mislocalization of the passenger protein to the matrix. Most mutations mapped within the first 81 amino-terminal residues of the cytochrome b2 moiety. They were located in three regions, all downstream of the matrix-targeting domain: a cluster of three basic residues upstream of the hydrophobic stretch, the hydrophobic stretch itself and the first residue of mature cytochrome b2. The level of missorting caused by mutations within the hydrophobic stretch did not correlate with their effects on hydrophobicity, but appeared to be related to changes in the conformation of this stretch. We conclude that the intermembrane space sorting signal of cytochrome b2 is decoded by protein-protein interactions rather than by simple partitioning into a lipid bilayer.
细胞色素b2是酵母线粒体外膜间隙的一种蛋白质，它由一个含80个残基的双功能前导序列合成。氨基末端部分类似于基质靶向信号。羧基末端部分作为外膜间隙的“分选信号”，含有一段疏水序列。为了确定这种分选信号，我们将细胞色素b2前体的前167个残基与一个报告蛋白融合，在酵母中表达该融合蛋白，并筛选导致报告蛋白错误定位于基质的突变。大多数突变位于细胞色素b2部分的前81个氨基末端残基内。它们位于三个区域，均在基质靶向结构域的下游：疏水序列上游的三个碱性残基簇、疏水序列本身以及成熟细胞色素b2的第一个残基。疏水序列内的突变导致的错误分选水平与其对疏水性的影响无关，而似乎与该序列构象的变化有关。我们得出结论，细胞色素b2的外膜间隙分选信号是通过蛋白质-蛋白质相互作用解码的，而不是通过简单地分配到脂质双层中。
