MOLECULAR CLONING AND EXPRESSING ANALYSIS OF CADM2B IN ADULT TISSUES OF GRASS CARP, CTENOPHARYNGODON IDELLUS

CADMs (cell adhesion molecules) family play important roles in establishing the first line of defense against illnesses and infections by mediating adhesion between immune mast cells and their target cells. Grass carp (Ctenopharyngodon idellus) has low livability at the age of 1 and 2 due to its susceptibility to infection by virus or bacterium. To investigate whether CADMs participate in building the first line of defense against infection in grass carp, we cloned the mRNA of grass carp cadm2b, and identify 4 different full length cDNA of cadm2b from grass carp brain tissue. According to sequences alignment, sequences of the 5' terminal are identical among all the four cDNAs while various deleted fragments found in three different position in their 3' terminals. These results indicated that these four mRNAs, named cadm2b, cadm2bX2, cadm2bX3 and cadm2bX6, are splicing variants from cadm2b gene. The full length of cadm2b is 1669 bp with a 1203 bp long open reading frame (ORF) coding 400 amino acids. The full length of cadm2bX2 is 2783 bp with a 1323 bp long ORF coding 440 amino acids. The full length of cadm2bX3 is 2755 bp with a 1296 bp long ORF coding 431 amino acids. The full length of cadm2bX6 cDNA is 2649 bp with a 1161 bp long ORF coding 386 amino acids. Prediction of amino acid sequences base on nucleotide sequenceds showed that all the four CADM2b isoforms contain the four conserve functional domains of CADM family in the N-terminals, but the C-terminals are variance. CADM2b has a juxtamembrane 4.1 protein binding domain and PDZ type Ⅱ protein binding domain in the C-terminal. Both CADM2bX2 and CADM2bX3 lack the PDZ type Ⅱ protein binding domain. CADM2bX6 possesses neither the juxtamembrane 4.1 protein binding domain nor the PDZ type Ⅱ protein binding domain. Quantitative RT-PCR results suggested that splicing variant cadm2b is the main form of cadm2b mRNA. A high-level cadm2b was detected in brain tissue and a very low-level cadm2b was detected in liver, kidney, heart and muscle. These findings suggest that CADM2b is a cell adhesion molecule synthesized and secreted by nonimmune cells, and might play a role in against various infections by mediating adhesion between immune mast cells and their target cells in grass carp.