A four-CRD C-type lectin from razor clam Sinonovacula constricta mediates agglutination and phagocytosis

C-type lectins are a superfamily of Ca²⁺-dependent carbohydrate-binding proteins that play crucial roles in invertebrate immunity. In this study, a novel C-type lectin gene (ScCTL-1) was identified in razor clam Sinonovacula constricta. The ScCTL-1 gene, consisting of four C-type carbohydrate recognition domains (CRDs) with an N-terminal signal peptide and a C-terminal transmembrane region. The gene is widely expressed in almost all tissues, with the highest expression in the hepatopancreas. To explore the functional characteristics of this structurally novel gene, tests of binding specificity, agglutinating activity, and phagocytic promoting activity were included in this study. Bacterial stimulation up-regulated ScCTL-1 expression in hemocytes. The binding activity of rScCTL-1 to bacteria was tested in vitro, and bacterial agglutination was observed under the same conditions. Ca²⁺ was essential for carbohydrate binding. Additionally, rScCTL-1 promoted the phagocytic activity of hemocytes to varying degrees against different bacteria, unlike the classical opsonin. These results suggest ScCTL-1 is a classical immune-related C-type lectin possessing unique immune-related properties.