Target antigens of malaria transmission blocking immunity exist as a stable membrane bound complex

Monoclonal antibodies (MoAbs) blocking transmission of malaria by blocking fertilization of female gametes by male gametes in the mosquito midgut immunoprecipitate three polypeptides (apparent non-reduced mol. wt of 230,000, 48,000, 45,000) from the surface of gametes and zygotes of Plasmodium gallinaceum and P. falciparum. Earlier studies have shown that the epitopes with which the MoAbs react are present only on the mol. wt 230,000 polypeptide in P. gallinaceum and the 48,000/45,000 mol. wt antigens in P. falciparum (Kumar 1985, Vermeulen et al. 1985). In gel permeation chromatography all three polypeptides were found to be co-eluted. Cross-linking reagents were used to cross-link proteins on the intact cells and in the soluble extracts. The cross-linked product immunoprecipitated by the MoAbs showed an approximate mol. wt of 290,000 to 300,000, suggesting a stoichiometry of 1:1 between the high and low mol. wt polypeptides. Evidence was also obtained for the existence of a subpopulation of the 48,000/45,000 mol. wt polypeptides which is not complexed with the 230,000 mol. wt polypeptide. These data indicate that the 230,000 polypeptide is physically associated with the 48,000 and 45,000 polypeptides; the high and low molecular weight polypeptides appear to exist in a stable membrane bound complex.