Gärtner F, Bömer U, Guiard B, Pfanner N
Biochemisches Institut, Universität Freiburg, Germany.
EMBO J. 1995 Dec 1;14(23):6043-57. doi: 10.1002/j.1460-2075.1995.tb00293.x.
Cytochrome b2 is imported into mitochondria and sorted to the intermembrane space by a bipartite N-terminal presequence, which is a matrix targeting sequenced followed by an intermembrane space sorting signal. The N-terminus of the mature protein forms a folded heme binding domain that depends on the unfoldase function of matrix (mt) Hsp70 for import. We report that the distance between the presequence and the heme binding domain is critical for the ability of mt-Hsp70 to promote import of the domain. Hybrid proteins with 40 or more amino acids between the presequence and the heme binding domain are arrested in the import machinery. The translocation arrest can be overcome by unfolding of the preprotein or by inactivation of the intermembrane space sorting signal. Moreover, the sorting signal prevents backsliding of the precursor polypeptide in the import site in the initial import step, when the signal has not made contact with the matrix. The results indicate that the sorting signal interacts with component(s) of the inner membrane/intermembrane space during the initial import step and promotes an early divergence of b2 preproteins from the general matrix import pathway, precluding an unfolding role for mt-Hsp70 in the translocation of most of the mature portions of a preprotein. We propose a sorting model of cytochrome b2 which explains the apparently divergent previous results by a unifying hypothesis.
细胞色素b2通过一个二分的N端前序列被导入线粒体并分选到膜间隙，该前序列是一个基质靶向序列，后面跟着一个膜间隙分选信号。成熟蛋白的N端形成一个折叠的血红素结合结构域，其导入依赖于基质（mt）Hsp70的解折叠酶功能。我们报道，前序列与血红素结合结构域之间的距离对于mt-Hsp70促进该结构域导入的能力至关重要。在前序列与血红素结合结构域之间有40个或更多氨基酸的杂合蛋白在导入机制中停滞。前体蛋白的解折叠或膜间隙分选信号的失活可以克服转运停滞。此外，分选信号在初始导入步骤中，当信号尚未与基质接触时，可防止前体多肽在导入位点回滑。结果表明，分选信号在初始导入步骤中与内膜/膜间隙的组分相互作用，并促进b2前体蛋白与一般基质导入途径的早期分化，排除了mt-Hsp70在前体蛋白大多数成熟部分转运中的解折叠作用。我们提出了一个细胞色素b2的分选模型，该模型通过一个统一的假说来解释先前明显不同的结果。
