Ci-AMBP: A highly conserved member of the microglobulin superfamily of proteinase inhibitors in grass carp, ctenopharyngodon idellus

A full-length cDNA clone encoding grass carp (Ctenopharyngodon idellus) α1-microglobulin/bikunin precursor (Ci-AMBP) was isolated by subtracted differential hybridization screening from a liver cDNA library. The deduced amino acid sequence shared approximately 50% sequence identity with its mammalian counterparts, but more than 90% identity with another fish species. AMBPs are the precursors of the plasma glycoproteins α1-microglobulin (α1m) and bikunin. Both peptide structures and their chromosomal organization were well conserved in Ci- AMBP. The α1m and bikunin polypeptides are separated by the typical tetrapeptide R-A-R-R that provides an endoproteolytic cleavage site for maturation. The genetic organization of domains and functional motifs indicated that Ci-AMBP is a typical member of the lipocalin and Kunitz-type protease inhibitor superfamilies. Expression of the Ci-AMBP gene in different tissues/organs was evaluated using semi-quantitative RT-PCR and, in contrast to the restricted expression in other species, transcripts were detected in a wide range of tissues. The most abundant expression occurred in the secretory organs, which supports the roles of α1m and bikunin in the immune response to diseases and in the stress response.