Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
Sci Rep. 2017 Jan 17;7:40738. doi: 10.1038/srep40738.
The β-barrel protein Tom40 and the α-helically anchored membrane protein Tom22 are the only universally conserved subunits of the protein translocase of the mitochondrial outer membrane (TOM). Tom22 has an N-terminal cytosolic and a C-terminal intermembrane space domain. It occurs in two variants: one typified by the yeast protein which has a cytosolic domain containing a cluster of acidic residues, and a shorter variant typified by the plant protein that lacks this domain. Yeast-type Tom22 functions as a secondary protein import receptor and is also required for the stability of the TOM complex. Much less is known about the more widespread short variant of Tom22, which is also found in the parasitic protozoan Trypanosoma brucei. Here we show that the intermembrane space domain of trypanosomal Tom22 binds mitochondrial precursor proteins and that it is essential for normal growth and mitochondrial protein import. Moreover, complementation experiments indicate that the intermembrane space domain cannot be replaced by the corresponding regions of the yeast or plant Tom22 orthologues. Lack or replacement of the short cytosolic domain, however, does not interfere with protein function. Finally, we show that only the membrane-spanning domain of trypanosomal Tom22 is essential for assembly of the trypanosomal TOM complex analogue.
β-桶状蛋白 Tom40 和 α-螺旋锚定的膜蛋白 Tom22 是线粒体膜外蛋白转位酶（TOM）唯一普遍保守的亚基。Tom22 具有一个 N 端胞质和一个 C 端膜间空间结构域。它有两种变体：一种以酵母蛋白为代表，其胞质结构域包含一簇酸性残基，另一种以植物蛋白为代表，缺乏这个结构域。酵母型 Tom22 作为二级蛋白输入受体发挥作用，对于 TOM 复合物的稳定性也是必需的。关于更为广泛的短变体 Tom22，我们了解得较少，这种变体也存在于寄生原生动物锥虫属。在这里，我们表明锥虫 Tom22 的膜间空间结构域与线粒体前体蛋白结合，对于正常生长和线粒体蛋白输入是必需的。此外，互补实验表明，该膜间空间结构域不能被酵母或植物 Tom22 同源物的相应区域取代。然而，短胞质结构域的缺失或替换并不干扰蛋白功能。最后，我们表明只有锥虫 Tom22 的跨膜结构域对于组装锥虫 TOM 复合物类似物是必需的。
