The structure of a tunicate C-type lectin from Polyandrocarpa misakiensis complexed with D-galactose
The structure of a tunicate C-type lectin from Polyandrocarpa misakiensis complexed with D-galactose
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DOI：
10.1006/jmbi.1999.2910
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发表时间：
1999-07-23
影响因子：
5.6
通讯作者：
Williams, RL
中科院分区：
文献类型：
--
作者：
Poget, SF;Legge, GB;Williams, RL
C型凝集素是钙依赖的碳水化合物识别蛋白。用等温滴定量热法测定了衣状多穿心果凝集素(TC14)的单链钙原子，其解离常数为2.6µM，证实了TC14对D-半乳糖及相关单糖的专一性。测定了聚穿心果凝集素与D-半乳糖络合的2.2埃X射线晶体结构。分析超速离心法表明，TC14在溶液中表现为二聚体。这反映在不对称单元中两个分子的存在，其中二聚体界面由两个N-末端β链的反平行配对和疏水相互作用形成。TC14采用典型的C型凝集素折叠，在结构上与其他C型凝集素不同，主要是在不同的环区和参与二聚体界面形成的第二个α-螺旋。D-半乳糖通过3和4-羟基氧原子与结合的钙原子配位而结合。蛋白质的丝氨酸、天冬氨酸和谷氨酸侧链与糖的3和4-羟基之间直接形成额外的氢键。比较TC14结合的半乳糖和大鼠甘露糖结合蛋白结合的甘露糖，揭示了这种凝集素是如何实现单糖专一性的。靠近结合部位的色氨酸侧链以及糖的3和4-羟基周围氢键受体和供体的分布是特异性的基本决定因素。然而，这些元件的排列方式与MBPA的工程半乳糖特异性突变体非常不同。从TC14在生理条件下是二聚体这一事实可以更容易地理解可能的生物学功能。(C)1999年学术出版社。
C-type lectins are calcium-dependent carbohydrate-recognising proteins. Isothermal titration calorimetry of the C-type Polyandrocarpa lectin (TC14) from the tunicate Polyandrocarpa misakiensis revealed the presence of a single calcium atom per monomer with a dissociation constant of 2.6 mu M, and confirmed the specificity of TC14 for D-galactose and related monosaccharides. We have determined the 2.2 Angstrom X-ray crystal structure of Polyandrocarpa lectin complexed with D-galactose. Analytical ultracentrifugation revealed that TC14 behaves as a dimer in solution. This is reflected by the presence of two molecules in the asymmetric unit with the dimeric interface formed by antiparallel pairing of the two N-terminal beta-strands and hydrophobic interactions. TC14 adopts a typical C-type lectin fold with differences in structure from other C-type lectins mainly in the diverse loop regions and in the second alpha-helix, which is involved in the formation of the dimeric interface. The D-galactose is bound through coordination of the 3 and 4-hydroxyl oxygen atoms with a bound calcium atom. Additional hydrogen bonds are formed directly between serine, aspartate and glutamate side-chains of the protein and the sugar 3 and 4-hydroxyl groups. Comparison of the galactose binding by TC14 with the mannose binding by rat mannose-binding protein reveals how monosaccharide specificity is achieved in this lectin. A tryptophan side-chain close to the binding site and the distribution of hydrogen-bond accepters and donors around the 3 and 4-hydroxyl groups of the sugar are essential determinants of specificity. These elements are, however, arranged in a very different way than in an engineered galactose-specific mutant of MBPA. Possible biological functions can more easily be understood from the fact that TC14 is a dimer under physiological conditions. (C) 1999 Academic Press.