Steiner H, Zollner A, Haid A, Neupert W, Lill R
Institut für Physiologische Chemie, Physikalische Biochemie und Zellbiologie, Universität München, Federal Republic of Germany.
J Biol Chem. 1995 Sep 29;270(39):22842-9. doi: 10.1074/jbc.270.39.22842.
Heme lyases are components of the mitochondrial intermembrane space facilitating the covalent attachment of heme to the apoforms of c-type cytochromes. The precursors of heme lyases are synthesized in the cytosol without the typical N-terminal mitochondrial targeting signal. Here, we have analyzed the mode of import and folding of the two heme lyases of the yeast Saccharomyces cerevisiae, namely of cytochrome c heme lyase and of cytochrome c1 heme lyase. For transport into mitochondria, both proteins use the general protein import machinery of the outer membrane. Import occurred independently of a membrane potential, delta psi, across the inner membrane and ATP in the matrix space, suggesting that the inner membrane is not required for transport along this direct sorting pathway. The presence of a large folded domain in heme lyases was utilized to study their folding in the intermembrane space. Formation of this domain occurred at the same rate as import, indicating that heme lyases fold either during or immediately after their transfer across the membrane. Folding was not affected by depletion of ATP and delta psi or by inhibitors of peptidylprolyl cis-trans isomerases, i.e. it does not involve homologs of known folding factors (like Hsp60 and Hsp70). The energy derived from folding cannot be regarded as a major driving force for import, since the folded domain could be imported into mitochondria with the same efficiency as the intact protein. We conclude that protein folding in the intermembrane space obeys principles different from those established for other subcellular compartments.
血红素裂解酶是线粒体外膜间隙的组成部分，有助于血红素与c型细胞色素的脱辅基形式共价结合。血红素裂解酶的前体在胞质溶胶中合成，没有典型的N端线粒体靶向信号。在此，我们分析了酿酒酵母的两种血红素裂解酶，即细胞色素c血红素裂解酶和细胞色素c1血红素裂解酶的导入和折叠模式。为了转运到线粒体中，这两种蛋白质都利用外膜的一般蛋白质导入机制。导入过程独立于跨内膜的膜电位Δψ和基质空间中的ATP，这表明沿此直接分选途径的转运不需要内膜。利用血红素裂解酶中一个大的折叠结构域来研究它们在外膜间隙中的折叠情况。该结构域的形成速率与导入速率相同，表明血红素裂解酶在跨膜转移过程中或转移后立即折叠。折叠不受ATP和Δψ耗竭或肽基脯氨酰顺反异构酶抑制剂的影响，即它不涉及已知折叠因子（如Hsp60和Hsp70）的同源物。由于折叠结构域可以与完整蛋白质以相同的效率导入线粒体，因此折叠产生的能量不能被视为导入的主要驱动力。我们得出结论，外膜间隙中的蛋白质折叠遵循与其他亚细胞区室不同的原则。
