Characterization of four spliced isoforms of a transmembrane C-type lectin from Procambarus clarkii and their function in facilitating WSSV infection

C-type lectin (CTL) is an important pattern recognition receptor that play vital functions in the innate immunity. Many soluble CTLs in crustacean participate in the inhibition or promotion of white spot syndrome virus (WSSV) infection. However, whether transmembrane CTLs participate in WSSV infection in crustacean remains unknown. In the present study, four spliced isoforms of a transmembrane CTL (designated as PcTlec) from Procambarus clarkii were identified for the first time. The genome structure of PcTlec contains eight exons, six known introns, and one unknown intron. PcTlec-isoform1 is produced by intron retention, whereas PcTlec-isoform3 and PcTlec-isoform4 are produced by exon skipping. All of them contain the transmembrane domain and characteristic carbohydrate recognition domain (CRD). Four PcTlec isoforms were mainly expressed in the hepatopancreas, stomach, and intestine. After WSSV challenge, the expression levels of PcTlec-isoform1-4 in the intestine were upregulated. The knockdown of the region shared by four PcTlec isoforms evidently decreased the expression of WSSV envelope protein VP28 and the copies of viral particles. A recombinant protein (rPcTlec-CRD) containing the CRD that was shared by four PcTlec isoforms was acquired by procaryotic expression system. The injection of purified rPcTlec-CRD protein evidently increased the VP28 expression and WSSV copies during viral infection. Moreover, rPcTlec-CRD could directly bind to WSSV and interact with VP28 protein. These findings indicate that new-found transmembrane CTL isoforms in P. clarkii may act as viral receptors that facilitate WSSV infection. This study contributes to the recognition and understanding of the functions of transmembrane CTLs in crustacean in the infection of host by WSSV.