Appenzeller-Herzog Christian, Roche Annie-Claude, Nufer Oliver, Hauri Hans-Peter
Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.
J Biol Chem. 2004 Mar 26;279(13):12943-50. doi: 10.1074/jbc.M313245200. Epub 2004 Jan 12.
The recycling mannose lectin ERGIC-53 operates as a transport receptor by mediating efficient endoplasmic reticulum (ER) export of some secretory glycoproteins. Binding of cargo to ERGIC-53 in the ER requires Ca2+. Cargo release occurs in the ERGIC, but the molecular mechanism is unknown. Here we report efficient binding of purified ERGIC-53 to immobilized mannose at pH 7.4, the pH of the ER, but not at slightly lower pH. pH sensitivity of the lectin was more prominent when Ca2+ concentrations were low. A conserved histidine in the center of the carbohydrate recognition domain was required for lectin activity suggesting it may serve as a molecular pH/Ca2+ sensor. Acidification of cells inhibited the association of ERGIC-53 with the known cargo cathepsin Z-related protein and dissociation of this glycoprotein in the ERGIC was impaired by organelle neutralization that did not impair the transport of a control protein. The results elucidate the molecular mechanism underlying reversible lectin/cargo interaction and establish the ERGIC as the earliest low pH site of the secretory pathway.
循环甘露糖凝集素ERGIC-53作为一种转运受体，通过介导一些分泌性糖蛋白从内质网（ER）高效输出发挥作用。货物在内质网中与ERGIC-53的结合需要Ca2+。货物释放发生在内质网-高尔基体中间腔（ERGIC），但其分子机制尚不清楚。在此，我们报告纯化的ERGIC-53在pH 7.4（内质网的pH值）时能有效结合固定化的甘露糖，而在略低的pH值时则不能。当Ca2+浓度较低时，凝集素的pH敏感性更为显著。碳水化合物识别结构域中心的一个保守组氨酸是凝集素活性所必需的，这表明它可能作为一个分子pH/Ca2+传感器。细胞酸化抑制了ERGIC-53与已知货物组织蛋白酶Z相关蛋白的结合，而细胞器中和损害了该糖蛋白在ERGIC中的解离，但不影响对照蛋白的转运。这些结果阐明了可逆凝集素/货物相互作用的分子机制，并确定ERGIC是分泌途径中最早的低pH位点。
