Stojanovski Diana, Guiard Bernard, Kozjak-Pavlovic Vera, Pfanner Nikolaus, Meisinger Chris
Institut für Biochemie und Molekularbiologie, Zentrum für Biochemie und Molekulare Zellforschung, Universität Freiburg, D-79104 Freiburg, Germany.
J Cell Biol. 2007 Dec 3;179(5):881-93. doi: 10.1083/jcb.200706043. Epub 2007 Nov 26.
The mitochondrial outer membrane contains two preprotein translocases: the general translocase of outer membrane (TOM) and the beta-barrel-specific sorting and assembly machinery (SAM). TOM functions as the central entry gate for nuclear-encoded proteins. The channel-forming Tom40 is a beta-barrel protein, whereas all Tom receptors and small Tom proteins are membrane anchored by a transmembrane alpha-helical segment in their N- or C-terminal portion. Synthesis of Tom precursors takes place in the cytosol, and their import occurs via preexisting TOM complexes. The precursor of Tom40 is then transferred to SAM for membrane insertion and assembly. Unexpectedly, we find that the biogenesis of alpha-helical Tom proteins with a membrane anchor in the C-terminal portion is SAM dependent. Each SAM protein is necessary for efficient membrane integration of the receptor Tom22, whereas assembly of the small Tom proteins depends on Sam37. Thus, the substrate specificity of SAM is not restricted to beta-barrel proteins but also includes the majority of alpha-helical Tom proteins.
外膜通用转位酶（TOM）和β-桶状蛋白特异性分选与组装机器（SAM）。TOM作为核编码蛋白的中央入口。形成通道的Tom40是一种β-桶状蛋白，而所有Tom受体和小Tom蛋白通过其N端或C端部分的跨膜α-螺旋段锚定在膜上。Tom前体的合成在胞质溶胶中进行，它们通过预先存在的TOM复合物进行导入。然后，Tom40的前体被转移到SAM进行膜插入和组装。出乎意料的是，我们发现C端带有膜锚的α-螺旋Tom蛋白的生物合成依赖于SAM。每个SAM蛋白对于受体Tom22的有效膜整合都是必需的，而小Tom蛋白的组装则依赖于Sam37。因此，SAM的底物特异性不仅限于β-桶状蛋白，还包括大多数α-螺旋Tom蛋白。
