Comparative Characterization of Sequence Structure, Gene Expression, and Enzyme Activity of Alpha-2-macroglobulin between Stichopus monotuberculatus (Stichopodidae) and Holothuria atra (Holothuriidae).

Alpha-2-macroglobulin ( A₂M) is essential for hosts defending against pathogen infections. In this study, two distinct A₂Ms were identified from sea cucumbers Stichopus monotuberculatus and Holothuria atra, which belong to different genera in the Phylum Echinodermata, and named StmA₂M and HaA₂M, respectively. Structural analysis of StmA₂M and HaA₂M revealed that all contain the bait region domain, thioester domain, and receptor-binding domain. Phylogenetic analysis indicated that StmA₂M and HaA₂M all belong to the echinoderm A₂Ms. The cleavage site RXXR and the catalytic histidine residue for complement component C3 were found in StmA₂M and HaA₂M. Compared with human A₂M, the three-dimensional structures of StmA₂M and HaA₂M were more similar to that of common carp A₂M. Both StmA₂M and HaA₂M showed the strongest response at transcriptional levels to lipopolysaccharide ( LPS) at 12 h and polyriboinosinic polyribocytidylic acid [Poly (I: C)] at 24 h, respectively. Activity of StmA₂M showed the strongest response to Poly (I: C) at 6 h, while the strongest change of HaA₂M activity was induced by LPS at 24 h. The differential mRNA expression and enzyme activity levels suggested that A₂M in sea cucumbers might be involved in different immune responses to LPS or Poly (I: C) challenge. [ABSTRACT FROM AUTHOR]