Makowski L, Caspar D L, Phillips W C, Goodenough D A
J Mol Biol. 1984 Apr 15;174(3):449-81. doi: 10.1016/0022-2836(84)90331-0.
X-ray diffraction patterns have been recorded from partially oriented specimens of gap junctions isolated from mouse liver and suspended in sucrose solutions of different concentration and thus of different electron density. Analysis of these diffraction patterns has shown that sucrose is excluded from the 6-fold rotation axis of the junction lattice for a length of about 100 A. This indicates that the aqueous channel of the junctions is in the closed, high resistance state in these preparations. Mapping of the sucrose-accessible space in the junction indicates that the cross-sectional area of the channel entrance on the cytoplasmic side of the membrane could be up to five times larger than the area of the transmembrane channel. Sucrose does not penetrate more than 20 A into the membrane along the channel. Apparently the aqueous channel, 8 to 10 A in radius for most of its length, is narrowed or blocked by a small feature about 50 A from the center of the gap. Very close interactions exist between the gap junction protein and the lipid polar head groups on the cytoplasmic surface of the membrane. In this region, the protein intercalates between the polar head groups. These results suggest that the gap junction protein may have a functional two-domain structure. One domain, with a molecular weight of about 15,000, spans one bilayer and half of the gap and is contained largely within a radius of 25 A from the 6-fold axis. The second domain is smaller and occupies the cytoplasmic surface of the gap junction membrane. Trypsin digestion removes about 4000 Mr from the cytoplasmic surface domain of the junction protein. Most of the material susceptible to trypsin digestion is located more than 28 A from the 6-fold axis.
已从小鼠肝脏分离出间隙连接的部分定向标本，并将其悬浮在不同浓度（因而电子密度不同）的蔗糖溶液中，记录了X射线衍射图谱。对这些衍射图谱的分析表明，蔗糖在连接晶格的六重旋转轴周围约100埃的长度范围内被排除在外。这表明在这些制剂中，连接的水通道处于关闭的高电阻状态。对连接中蔗糖可及空间的映射表明，膜细胞质侧通道入口的横截面积可能比跨膜通道的面积大五倍之多。蔗糖沿通道向膜内渗透不超过20埃。显然，水通道在其大部分长度上半径为8至10埃，在距间隙中心约50埃处被一个小特征变窄或阻断。间隙连接蛋白与膜细胞质表面的脂质极性头部基团之间存在非常紧密的相互作用。在该区域，蛋白质插入极性头部基团之间。这些结果表明，间隙连接蛋白可能具有功能性的双结构域结构。一个结构域分子量约为15,000，跨越一个双层和间隙的一半，主要包含在距六重轴25埃半径范围内。第二个结构域较小，占据间隙连接膜的细胞质表面。胰蛋白酶消化从连接蛋白的细胞质表面结构域去除约4000道尔顿。大多数易受胰蛋白酶消化的物质位于距六重轴超过28埃处。
