Molecular cloning and expression analysis of α2-macroglobulin in the kuruma shrimp, Marsupenaeus japonicus

The cDNA encoding the kuruma shrimp, Marsupenaeus japonicus α2-macroglobulin (α2M) was obtained by screening a haemocyte cDNA library and 5′ RACE PCR amplification. The full length cDNA of 4748 bp contains an open reading frame of 4518 nucleotides that translates into a 1505-amino acid putative peptide, with a 5′untranslated region (UTR) of 59 bp and a 3′UTR of 171 bp. The open reading frame encodes an N-terminal signal sequence of 17 residues and a mature protein of 1488 residues. The entire amino acid sequence is similar to the α2M sequences of arthropods (30–31% identity), mammals (26–27% identity) and fish (25–28% identity). The M. japonicus α2M sequence contains putative functional domains including a bait region, an internal thiol ester site, and a receptor-binding domain, which are present in mammalian α2Ms. In a healthy shrimp, the mRNA of α2M was mainly expressed in haemocytes. In addition, the expression level of α2M mRNA was dramatically increased by through time upon oral administration of peptidoglycan (PG), which is an immune stimulant. The highest expression of α2M mRNA was observed 7 days after feeding with PG. These results suggest that the shrimp α2M is an important molecule in immune system.