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