CRYSTAL-STRUCTURE OF A NONTOXIC MUTANT OF HEAT-LABILE ENTEROTOXIN, WHICH IS A POTENT MUCOSAL ADJUVANT

Two closely related bacterial toxins, heat-labile enterotoxin (LT-I) a nd cholera toxin (CT), not only invoke a toxic activity that affects m any victims worldwide but also contain a beneficial mucosal adjuvant a ctivity that significantly enhances the potency of vaccines in general , For the purpose of vaccine design it is most interesting that the un desirable toxic activity of these toxins can be eliminated by the sing le-site mutation Ser63Lys in the A subunit while the mucosal adjuvant activity is still present. The crystal structure of the Ser63Lys mutan t of LT-I is determined at 2.0 Angstrom resolution. Its structure appe ars to be essentially the same as the wild-type LT-I structure. The su bstitution Ser63Lys was designed, based on the wild-type LT-I crystal structure, to decrease toxicity by interfering with NAD binding and/or catalysis. In the mutant crystal structure, the newly introduced lysi ne side chain is indeed positioned such that it could potentially obst ruct the productive binding mode of the substrate NAD while at the sam e time its positive charge could possibly interfere viith the critical function of nearby charged groups in the active site of LT-I. The fac t that the Ser63Lys mutant of LT-I does not disrupt the wild-type LT-I structure makes the non-toxic mutant potentially suitable, from a str uctural point of view, to be used as a vaccine to prevent enterotoxige nic E. coli infections. The structural similarity of mutant and wild-t ype toxin might also be the reason why the inactive Ser63Lys variant r etains its adjuvant activity.