Focus on molecules: major intrinsic protein.
Focus on molecules: major intrinsic protein.
复制标题
关注分子：主要内在蛋白质。
DOI：
10.1016/j.exer.2010.11.011
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发表时间：
2012
影响因子：
3.4
通讯作者：
Shiels,Alan
中科院分区：
文献类型：
--
作者：
Shiels,Alan
人和小鼠的透镜纤维主要内在蛋白（MIP）基因分别位于染色体12 q13（MIP;基因ID：4284）和染色体10（Mip;基因ID：17339）。每个基因包含四个外显子，编码263个氨基酸（Mr~ 28-kDa）的疏水性跨膜蛋白。MIP是水通道的水通道蛋白（AQP）家族的成员（pfam 00230），也称为水通道蛋白-0（AQP 0）（Chepelinsky 2008综述）。1984年，牛MIP cDNA的首次克隆和序列分析预测了具有六个跨质膜α-螺旋的单体蛋白拓扑结构（图1）。随后发现MIP与水通道蛋白具有同源性，这导致了“沙漏”结构模型，反映了水通道蛋白基因家族中关键的基因复制事件。发现MIP单体含有两个串联重复的内部结构对称性，每个串联重复含有三个跨膜螺旋的氨基和羧基末端“半孔”和具有高度保守的Asn-Pro-Ala（NPA）基序的疏水环（环B和E），其折叠到膜中以形成功能性水孔。X射线和电子晶体学技术已经在很大程度上证实了MIP单体的沙漏结构，并进一步证实MIP形成均四聚体，这与通过透镜核心或核中的纤维细胞膜的冷冻断裂电子显微镜观察到的正方形阵列一致。除了四聚体的单层2D晶体之外，从透镜核纯化的MIP可以形成MIP八聚体的双层2D晶体（即，在MIP四聚体-MIP四聚体接合处配准），其具有与通过原位透镜纤维细胞膜的薄片透射电子显微镜观察到的突出的“薄接合处”（11-14 nm）相似的尺寸。“非连接性”MIP四聚体向“连接性”MIP八聚体的转化与胞质氨基和羧基末端的蛋白水解截短以及随后通过并置四聚体的胞外环（A和C）之间的脯氨酸介导的疏水相互作用的稳定化有关（图1）。然而，原位薄连接的免疫电子显微镜显示，在并置的纤维细胞膜中，MIP的大部分区域是不对齐的（即MIP四聚体-
The human and mouse genes for lens fiber major intrinsic protein (MIP) reside on chromosome 12q13 (MIP; Gene ID: 4284) and chromosome 10 (Mip; Gene ID: 17339), respectively. Each gene comprises four exons encoding a hydrophobic, transmembrane protein of 263-amino-acids (Mr~ 28-kDa). MIP is a member of the aquaporin (AQP) family of water channels (pfam00230) and also referred to as aquaporin-0 (AQP0)(reviewed by Chepelinsky 2008).The first cloning and sequence analysis of a bovine MIP cDNA in 1984 predicted a monomeric protein topology with six plasma membrane-spanning α-helices (Fig. 1). The subsequent discovery that MIP shared homology with aquaporins led to the “hourglass” structural model which reflected a key gene-duplication event in the aquaporin gene family. MIP monomers were found to contain an internal structural symmetry of two tandem repeats, each containing an amino-and carboxy-terminal “hemi-pore” of three transmembrane-helices and a hydrophobic-loop (loops B and E) with a highly conserved Asn-Pro-Ala (NPA) motif that fold into the membrane to form a functional water pore. X-ray and electron crystallography techniques have largely substantiated the hourglass structure of MIP monomers, and further confirmed that MIP forms homo-tetramers, consistent with the square-arrays observed by freeze-fracture electron microscopy of fiber cell membranes in the lens core or nucleus. In addition to single-layered 2D crystals of tetramers, MIP purified from the lens core can form double-layered 2D crystals of MIP octamers (ie in register MIP tetramer-MIP tetramer junctions) with similar dimensions to the prominent “thin-junctions”(11–14 nm) observed by thin-section transmission electron microscopy of lens fiber cell membranes in situ. Conversion of “non-junctional” MIP tetramers to “junctional” MIP octamers has been associated with proteolytic truncation of the cytoplasmic amino-and carboxy-termini, and subsequent stabilization by prolinemediated hydrophobic interactions between the extracellular loops (A and C) of apposed tetramers (Fig. 1). However, immuno-electron microscopy of thin-junctions in situ reveals mostly out-of-register domains of MIP in apposing fiber cell membranes (ie MIP tetramer-