Lim Koh-Pang, Hong Wanjin
Membrane Biology Laboratory, Institute of Molecular and Cell Biology, Proteos Building, 61 Biopolis Drive, Singapore 138673, Singapore.
J Biol Chem. 2004 Dec 24;279(52):54770-82. doi: 10.1074/jbc.M411315200. Epub 2004 Oct 8.
Around 50 mammalian and 15 yeast proteins are known to contain the phox (PX) domain, the majority (about 30) of which is classified as sorting nexins (SNXs). The PX domain, a hallmark of these proteins, is a conserved stretch of about 120 amino acids and is recently shown to mediate phosphoinositide binding. A few PX domain proteins (including some SNXs) have been shown to participate in diverse cellular processes such as protein sorting, signal transduction, and vesicle fusion. In this report, we present our results supporting a role of human IRAS to act as a SNX. The mouse homologue, previously identified as Nischarin, has been shown to interact with the alpha(5) subunit of integrin and inhibit cell migration (Alahari, S. K., Lee J. W., and Juliano R. L. (2000) J. Cell Biol. 51, 1141-1154). Its human homologue (imidazoline receptor antisera-selected (IRAS)), on the other hand, contains an NH(2)-terminal extension and is a larger protein of 1504 amino acids consisting of an NH(2)-terminal PX domain, 5 putative leucine-rich repeats, a predicted coiled-coil domain, and a long COOH-terminal region. We show that it has the ability to homo-oligomerize via its coiled-coil region. The PX domain of IRAS is essential for association with phosphatidylinositol 3-phosphate-enriched endosomal membranes. However, the PX domain of IRAS alone is insufficient for its localization to endosomes, unless the coiled-coil domain was included or it is artificially dimerized by glutathione S-transferase. Interaction of human IRAS with alpha(5) integrin is not affected by the NH(2)-terminal extension, and overexpression of IRAS could cause a redistribution of surface alpha(5) integrin to intracellular endosomal structures.
已知约有50种哺乳动物蛋白和15种酵母蛋白含有PX（phox）结构域，其中大多数（约30种）被归类为分选连接蛋白（SNXs）。PX结构域是这些蛋白的一个标志，是一段约120个氨基酸的保守序列，最近显示它能介导磷酸肌醇结合。一些含PX结构域的蛋白（包括一些SNXs）已被证明参与多种细胞过程，如蛋白质分选、信号转导和囊泡融合。在本报告中，我们展示了支持人IRAS作为一种SNX发挥作用的结果。小鼠同源物先前被鉴定为Nischarin，已被证明能与整合素的α(5)亚基相互作用并抑制细胞迁移（阿拉哈里，S.K.，李J.W.，和朱利亚诺R.L.（2000年）《细胞生物学杂志》51卷，1141 - 1154页）。另一方面，它的人同源物（咪唑啉受体抗血清选择蛋白（IRAS））含有一个NH(2) - 末端延伸序列，是一个由1504个氨基酸组成的较大蛋白，由一个NH(2) - 末端PX结构域、5个假定的富含亮氨酸重复序列、一个预测的卷曲螺旋结构域和一个长的COOH - 末端区域组成。我们表明它有能力通过其卷曲螺旋区域进行同源寡聚化。IRAS的PX结构域对于与富含磷脂酰肌醇3 - 磷酸的内体膜结合至关重要。然而，单独的IRAS的PX结构域不足以使其定位于内体，除非包含卷曲螺旋结构域或通过谷胱甘肽S - 转移酶将其人为二聚化。人IRAS与α(5)整合素的相互作用不受NH(2) - 末端延伸序列的影响，并且IRAS的过表达可能导致表面α(5)整合素重新分布到细胞内的内体结构中。
