Comparison of the thioester domain and adjacent regions of the alpha2-macroglobulin from different South Atlantic crustaceans

Abstract

Alpha2-macroglobulin;Thioester domain;Crustacean immunity;Molecular cloning;Proteinase inhibitorAlpha2-macroglobulin(a2M)isabroad-spectrumprotein-ase inhibitor that belongs to the group of the thioesterplasmaproteinsuperfamily,whichincludesalsothecomple-mentcomponentsC3,C4andC5andthehuman’spregnancyzone protein. This protein superfamily contains in its struc-tureacharacteristicinternalreactivethioesterbond(exceptC5) that links cysteinyl and glutamyl amino acid residues[1,2]. The a2M inhibition mechanism starts with the pro-teolytic attack of its ‘‘bait region’’ by a proteinase. Thisbait region has target sites for all proteinases and afterproteolysis, the native a2M molecule goes through a confor-mational change that culminates with the formation ofamolecularcagethatentrapstheboundproteinase.Asacon-sequence of this structural change, the a2M internal thio-ester bond is also cleaved and exposes its cysteinyl thioland carbonyl groups of glutamic acid. The carbonyl groupsare highly reactive and can form covalent crosslinks with3-amino groups of the lysines of the encaged proteinase [2].The first reports on an a2M-like protein in crustaceansdate from the late 1980s. This protein was biochemicallypurified and characterised from the haemolymph of thelobster Homarus americanus and the crayfish Pacifastacusleniusculus [3,4]. However, after these first observations,very few other studies were done with the a2M of crusta-ceans. The first cDNA encoding for a2M was reported inthe kuruma prawn Marsupenaeus japonicus and was