Purification and characterization of alpha2-macroglobulin from grass carp Ctenopharyngodon idellus: cloning a segment of the corresponding gene

The plasma protein alpha2-macroglobulin (alpha2M) was purified by gel filtration and anion-exchange chromatography from grass carp plasma. The alpha2M consists of two different subunits of molecular weight 95 kDa and 80 kDa, respectively. The characteristics of grass carp alpha2M are similar to mammalian alpha2M, in that grass carp alpha2M exists in two forms: a fast-form and a slow-form. The former is complexed with protease. The sequence of grass carp alpha2M-conserved region and a region containing the bait region was determined by sequence analysis using polymerase chain reaction (PCR). The deduced amino acid sequence of the conserved region is similar to the alpha2M sequence of common carp, however, the bait region amino acid sequence is dramatically distinct from that of common carp. This may partially explain the differential ability of alpha2M of different species to inhibit different proteases. The alpha2M was able to inhibit Aeromonas hydrophila extracellular protease (AhECPase) and thus may play a role in resistance to infection by this pathogen.