Characterization of the βγ-crystallin domains of βγ-CAT, a non-lens βγ-crystallin and trefoil factor complex, from the skin of the toad Bombina maxima

Abstract

βγ-CAT is a naturally existing 72-kDa complex of a non-lens βγ-crystallin (α-subunit, CAT-α) and a trefoil factor (β-subunit, CAT-β) that contains a non-covalently linked form of αβ(2) and was isolated from the skin secretions of the toad Bombina maxima. The N-terminal region of CAT-α (CAT-αN, residues 1-170) contains two βγ-crystallin domains while the C-terminal region (CAT-αC) has sequence homology to the membrane insertion domain of the Clostridium perfringens epsilon toxin. To examine the biochemical characteristics of the βγ-crystallin domains of βγ-CAT, CAT-αN, CAT-αC and CAT-β were expressed in Escherichia coli. Co-immunoprecipitation of the naturally assembled βγ-CAT confirmed that the CAT-α and CAT-β complex always exists. Furthermore, recombinant CAT-β bound recombinant CAT-αN. Ca(2+)-binding motifs were identified in CAT-αN, and recombinant CAT-αN was able to bind the calcium probe terbium. However, the conformation of CAT-αN was not significantly altered upon Ca(2+) binding. βγ-CAT possesses strong hemolytic activity toward human erythrocytes, and treatment of erythrocytes with βγ-CAT resulted in a rapid Ca(2+) influx, eventually leading to hemolysis. However, in the absence of extracellular Ca(2+), no significant hemolysis was detected, even though the binding and oligomerization of βγ-CAT in the erythrocyte membrane was observed. Our data demonstrate the binding of CAT-β (a trefoil factor) to CAT-αN (βγ-crystallin domains) and provide a basis for the formation of a βγ-crystallin and trefoil factor complex in vivo. Furthermore, the βγ-crystallin domains of βγ-CAT are able to bind Ca(2+), and βγ-CAT-induced hemolysis is Ca(2+) dependent.