Structural Basis of Carbohydrate Recognition by Calreticulin
Structural Basis of Carbohydrate Recognition by Calreticulin
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DOI：
10.1074/jbc.m110.168294
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发表时间：
2010-12-03
影响因子：
4.8
通讯作者：
Gehring, Kalle
中科院分区：
文献类型：
--
作者：
Kozlov, Guennadi;Pocanschi, Cosmin L.;Gehring, Kalle
钙连接蛋白循环是糖基化蛋白通过与膜蛋白钙连接蛋白（CNX）或其可溶性同源物钙网蛋白（CRT）结合而在内质网腔中经历折叠循环的过程。CNX和CRT特异性识别单葡萄糖基化Glc（1）Man（9）GlcNAc（2）聚糖，但这种特异性背后的结构决定因素尚不清楚。在这里，我们报告了CRT凝集素结构域与四糖α-Glc（1 -> 3）-α-Man-（1 -> 2）-α-Man-（1-> 2）-Man复合的1.95埃晶体结构。四糖结合到CRT上由凹形β-折叠形成的长通道上。所有四个糖部分通过广泛的氢键和疏水接触网络参与蛋白质结合。该结构解释了在碳水化合物的非还原端需要葡萄糖;葡萄糖的氧O-2完全适合CRT侧链形成的口袋，同时与Gly（124）的羰基和Lys（111）的侧链形成直接氢键。该结构还解释了CRT/CNX中Cys（105）-Cys（137）二硫键对于有效碳水化合物结合的需求。Cys（105）-Cys（137）二硫键与Glc（1）Man（3）四糖的第三和第四糖部分密切接触。最后，结构合理化以前的诱变CRT和奠定了结构基础，为未来的研究CNX/CRT在不同的生物途径中的作用。
The calnexin cycle is a process by which glycosylated proteins are subjected to folding cycles in the endoplasmic reticulum lumen via binding to the membrane protein calnexin ( CNX) or to its soluble homolog calreticulin ( CRT). CNX and CRT specifically recognize monoglucosylated Glc(1)Man(9)GlcNAc(2) glycans, but the structural determinants underlying this specificity are unknown. Here, we report a 1.95-angstrom crystal structure of the CRT lectin domain in complex with the tetrasaccharide alpha-Glc(1 -> 3)-alpha-Man-(1 -> 2)-alpha-Man-(1 -> 2)-Man.The tetrasaccharide binds to a long channel on CRT formed by a concave beta-sheet. All four sugar moieties are engaged in the protein binding via an extensive network of hydrogen bonds and hydrophobic contacts. The structure explains the requirement for glucose at the nonreducing end of the carbohydrate; the oxygen O-2 of glucose perfectly fits to a pocket formed by CRT side chains while forming direct hydrogen bonds with the carbonyl of Gly(124) and the side chain of Lys(111). The structure also explains a requirement for the Cys(105)-Cys(137) disulfide bond in CRT/CNX for efficient carbohydrate binding. The Cys(105)-Cys(137) disulfide bond is involved in intimate contacts with the third and fourth sugar moieties of the Glc(1)Man(3) tetrasaccharide. Finally, the structure rationalizes previous mutagenesis of CRT and lays a structural groundwork for future studies of the role of CNX/CRT in diverse biological pathways.