Comparison of coligenoid formation by B subunits of porcine and human Escherichia coli heat-labile enterotoxins

A hybrid B subunit (coligenoid) of heat-labile enterotoxin could not be made from human heat-labile enterotoxin B subunit(LTh-B) and porcine LTp-B subunit(LTp-B). LTp-B monomer was able to form coligenoid by reassociation with homologous LTp-B monomer, but not with heterogeneous LTh-B monomer and vice versa. The dissociation of both coligenoids into monomers by SDS treatment occurred in a time-dependent manner, but the dissociation of LTh-B colligenoid was faster than that of LTp-B coligenoid. The association of LTp-B monomer is tighter than that of LTh-B monomer. The pI values of LTp-B coligenoid, LTp-B monomer and denatured LTp-B monomer were similar at 9.6-9.8, while the pI values of LTh-B coligenoid, LTh-B monomer and denatured LTh-B monomer were determined as 5.6-5.8, 9.2-9.6 and 9.2-9.6, respectively. All the ionic amino acids of LTp-B exist on the coligenoid surface. The difference in pI values between LTh-B coligenoid and LTh-B monomer suggests that some basic amino acids are located within the LTh-B coligenoid complex, but are exposed in the LTh-B monomer. These data suggest that the 4 amino acid substitutions between LTh-B and LTp-B result in a three dimensional structure difference and a less stable formation of LTh-B coligenoid compared to LTp-B coligenoid.