Larue Kane, Kimber Matthew S, Ford Robert, Whitfield Chris
Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
J Biol Chem. 2009 Mar 13;284(11):7395-403. doi: 10.1074/jbc.M809068200. Epub 2009 Jan 7.
Lipopolysaccharide (LPS) is a major component of the Gram-negative outer membrane and is an important virulence determinant. The O-antigen polysaccharide of the LPS molecule provides protection from host defenses, and the length of O-antigen chains plays a pivotal role. In the Wzy-dependent O-antigen biosynthesis pathway, the integral inner membrane protein Wzz determines the O-antigen chain length. How these proteins function is currently unknown, but the hypothesis includes activities such as a "molecular ruler" or a "molecular stopwatch," and other possibilities may exist. Wzz homologs are membrane proteins with two transmembrane helices that flank a large periplasmic domain. Recent x-ray crystallographic studies of the periplasmic portions of Wzz proteins found multiple oligomeric forms, with quaternary structures favoring the "molecular ruler" interpretation. Here, we have studied full-length Wzz proteins with the transmembrane portions embedded in lipid membranes. Using electron microscopy and image analysis we find a unique hexameric state rather than differing oligomeric forms. The data suggest that in vivo Wzz proteins determine O-antigen chain length via subtle structure-function relationships at the level of primary, secondary, or tertiary structure within the context of a hexameric complex.
脂多糖（LPS）是革兰氏阴性菌外膜的主要成分，也是一种重要的毒力决定因素。LPS分子的O抗原多糖可保护细菌免受宿主防御，O抗原链的长度起着关键作用。在依赖Wzy的O抗原生物合成途径中，整合内膜蛋白Wzz决定O抗原链的长度。目前尚不清楚这些蛋白质是如何发挥作用的，但假说包括“分子尺”或“分子秒表”等活动，可能还存在其他可能性。Wzz同源物是具有两个跨膜螺旋的膜蛋白，这两个跨膜螺旋位于一个大的周质结构域两侧。最近对Wzz蛋白周质部分的x射线晶体学研究发现了多种寡聚形式，其四级结构支持“分子尺”的解释。在这里，我们研究了跨膜部分嵌入脂质膜中的全长Wzz蛋白。通过电子显微镜和图像分析，我们发现了一种独特的六聚体状态，而不是不同的寡聚形式。数据表明，在体内，Wzz蛋白通过六聚体复合物背景下一级、二级或三级结构水平上微妙的结构-功能关系来决定O抗原链的长度。
