Chan Nickie C, Lithgow Trevor
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Parkville, Victoria 3010, Australia.
Mol Biol Cell. 2008 Jan;19(1):126-36. doi: 10.1091/mbc.e07-08-0796. Epub 2007 Oct 31.
The sorting and assembly machinery (SAM) complex functions in the assembly of beta-barrel proteins into the mitochondrial outer membrane. It is related to the Omp85/YaeT machinery in bacterial outer membranes, but the eukaryotic SAM complex is distinguished by two peripheral subunits, Sam37 and Sam35, that sit on the cytosolic face of the complex. The function of these subunits in beta-barrel protein assembly is currently unclear. By screening a library of sam35 mutants, we show that 13 distinct alleles were each specifically suppressed by overexpression of SAM37. Two of these mutants, sam35-409 and sam35-424, show distinct phenotypes that enable us to distinguish the function of Sam35 from that of Sam37. Sam35 is required for the SAM complex to bind outer membrane substrate proteins: destabilization of Sam35 inhibits substrate binding by Sam50. Sam37 acts later than Sam35, apparently to assist release of substrates from the SAM complex. Very different environments surround bacteria and mitochondria, and we discuss the role of Sam35 and Sam37 in terms of the problems peculiar to mitochondrial protein substrates.
分选与组装机制（SAM）复合体在β-桶状蛋白组装到线粒体外膜的过程中发挥作用。它与细菌外膜中的Omp85/YaeT机制相关，但真核生物的SAM复合体的特点是有两个位于复合体胞质面的外周亚基Sam37和Sam35。目前尚不清楚这些亚基在β-桶状蛋白组装中的功能。通过筛选sam35突变体文库，我们发现13个不同的等位基因各自被SAM37的过表达特异性抑制。其中两个突变体sam35 - 409和sam35 - 424表现出不同的表型，这使我们能够区分Sam35和Sam37的功能。SAM复合体结合外膜底物蛋白需要Sam35：Sam35的失稳会抑制Sam50与底物的结合。Sam37的作用比Sam35晚，显然是协助底物从SAM复合体释放。细菌和线粒体所处的环境截然不同，我们从线粒体蛋白底物特有的问题角度讨论了Sam35和Sam37的作用。
