Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Minato-ku, Tokyo, 108-8477, Japan.
Sci Rep. 2017 Apr 4;7:45818. doi: 10.1038/srep45818.
C-type lectins (CTLs) are calcium-dependent carbohydrate-binding proteins known to assist the innate immune system as pattern recognition receptors (PRRs). The binding specificity of CTLs lies in the motif of their carbohydrate recognition domain (CRD), the tripeptide motifs EPN and QPD bind to mannose and galactose, respectively. However, variants of these motifs were discovered including a QAP sequence reported in shrimp believed to have the same carbohydrate specificity as QPD. Here, we characterized a novel C-type lectin (MjGCTL) possessing a CRD with a QAP motif. The recombinant MjGCTL has a calcium-dependent agglutinating capability against both Gram-negative and Gram-positive bacteria, and its sugar specificity did not involve either mannose or galactose. In an encapsulation assay, agarose beads coated with rMjGCTL were immediately encapsulated from 0 h followed by melanization at 4 h post-incubation with hemocytes. These results confirm that MjGCTL functions as a classical CTL. The structure of QAP motif and carbohydrate-specificity of rMjGCTL was found to be different to both EPN and QPD, suggesting that QAP is a new motif. Furthermore, MjGCTL acts as a PRR binding to hemocytes to activate their adherent state and initiate encapsulation.
C 型凝集素 (CTLs) 是一种依赖钙的碳水化合物结合蛋白，已知其作为模式识别受体 (PRRs) 辅助先天免疫系统。CTLs 的结合特异性在于其碳水化合物识别域 (CRD) 的基序，三肽基序 EPN 和 QPD 分别与甘露糖和半乳糖结合。然而，已经发现了这些基序的变体，包括在虾中报道的具有与 QPD 相同碳水化合物特异性的 QAP 序列。在这里，我们表征了一种具有 QAP 基序的新型 C 型凝集素 (MjGCTL)。重组 MjGCTL 具有对革兰氏阴性和革兰氏阳性细菌的钙依赖性凝集能力，其糖特异性不涉及甘露糖或半乳糖。在包封测定中，琼脂糖珠在与血细胞孵育 4 小时后立即被 rMjGCTL 包封，随后发生黑化。这些结果证实 MjGCTL 作为一种经典的 CTL 发挥作用。发现 rMjGCTL 的 QAP 基序的结构和碳水化合物特异性与 EPN 和 QPD 都不同，表明 QAP 是一个新的基序。此外，MjGCTL 作为一种与血细胞结合的 PRR 发挥作用，激活其附着状态并启动包封。
