Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, United Kingdom.
Schools of Cellular and Molecular Medicine and Biochemistry, Medical Sciences Building, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom.
J Biol Chem. 2011 Sep 2;286(35):30606-30614. doi: 10.1074/jbc.M111.243865. Epub 2011 Jul 5.
MxiG is a single-pass membrane protein that oligomerizes within the inner membrane ring of the Shigella flexneri type III secretion system (T3SS). The MxiG N-terminal domain (MxiG-N) is the predominant cytoplasmic structure; however, its role in T3SS assembly and secretion is largely uncharacterized. We have determined the solution structure of MxiG-N residues 6-112 (MxiG-N(6-112)), representing the first published structure of this T3SS domain. The structure shows strong structural homology to forkhead-associated (FHA) domains. Canonically, these cell-signaling modules bind phosphothreonine (Thr(P)) via highly conserved residues. However, the putative phosphate-binding pocket of MxiG-N(6-112) does not align with other FHA domain structures or interact with Thr(P). Furthermore, mutagenesis of potential phosphate-binding residues has no effect on S. flexneri T3SS assembly and function. Therefore, MxiG-N has a novel function for an FHA domain. Positioning of MxiG-N(6-112) within the EM density of the S. flexneri needle complex gives insight into the ambiguous stoichiometry of the T3SS, supporting models with 24 MxiG subunits in the inner membrane ring.
MxiG 是一种单次跨膜蛋白，在福氏志贺菌 III 型分泌系统 (T3SS) 的内膜环内寡聚化。MxiG 的 N 端结构域（MxiG-N）是主要的细胞质结构；然而，其在 T3SS 组装和分泌中的作用在很大程度上尚未确定。我们已经确定了 MxiG-N 残基 6-112（MxiG-N（6-112））的溶液结构，这代表了该 T3SS 结构域的第一个发表结构。该结构显示出与 forkhead-associated（FHA）结构域的强烈结构同源性。通常，这些细胞信号模块通过高度保守的残基结合磷酸苏氨酸 (Thr(P))。然而，MxiG-N（6-112）的假定磷酸结合口袋与其他 FHA 结构域结构不匹配，也不与 Thr(P)相互作用。此外，潜在磷酸结合残基的突变对福氏志贺菌 T3SS 组装和功能没有影响。因此，MxiG-N 具有 FHA 结构域的新功能。MxiG-N（6-112）在福氏志贺菌针复合物的 EM 密度中的定位提供了对 T3SS 模糊的分子量的深入了解，支持了内膜环中有 24 个 MxiG 亚基的模型。
