Functional Mapping of the Lectin Activity Site on the β-Prism Domain of Vibrio cholerae Cytolysin IMPLICATIONS FOR THE MEMBRANE PORE-FORMATION MECHANISM OF THE TOXIN

Vibrio cholerae cytolysin (VCC) is a prominent member in the family of beta-barrel pore-forming toxins. It induces lysis of target eukaryotic cells by forming transmembrane oligomeric beta-barrel channels. VCC also exhibits prominent lectin-like activity in interacting with beta 1-galactosyl-terminated glycoconjugates. Apart from the cytolysin domain, VCC harbors two lectin-like domains: the beta-Trefoil and the beta-Prism domains; however, precise contribution of these domains in the lectin property of VCC is not known. Also, role(s) of these lectin-like domains in the mode of action of VCC remain obscure. In the present study, we show that the beta-Prism domain of VCC acts as the structural scaffold to determine the lectin activity of the protein toward beta 1-galactosyl-terminated glycoconjugates. Toward exploring the physiological implication of the beta-Prism domain, we demonstrate that the presence of the beta-Prism domain-mediated lectin activity is crucial for an efficient interaction of the toxin toward the target cells. Our results also suggest that such lectin activity may act to regulate the oligomerization ability of the membrane-bound VCC toxin. Based on the data presented here, and also consistent with the existing structural information, we propose a novel mechanism of regulation imposed by the beta-Prism domain's lectin activity, implicated in the process of membrane pore formation by VCC.