MECHANISM OF REGULATION OF MALARIAL INVASION BY EXTRAERYTHROCYTIC LIGANDS

Invasion of red cells by Plasmodium falciparum in vitro was inhibited by a range of extracellular ligands, none of which block the major rec eptors for merozoites. Most effective, in terms of dose response, were two monoclonal antibodies against the Wr(b) antigen on glycophorin A; wheat germ agglutinin which also binds to glycophorin, and an anti-ba nd 3 monoclonal antibody, caused inhibition of invasion at higher leve ls of saturation, while concanavalin A, which binds to band 3, was wit hout effect. Ah the ligands except concanavalin A, increased the rigid ity of the host cell membrane. The anti-Wr(b) antibodies generated the highest dose response effect, but no correlation between invasion and shear elastic modulus of the membrane could be established. All ligan ds, with the exception of concanavalin A, caused a reduction in the tr anslationally mobile fractions of band 3 and glycophorin, as revealed by fluorescence recovery after photobleaching (FRAP). Invasion diminis hed with loss of mobile band 3, engendered by bound wheat germ aggluti nin or anti-band 3, falling precipitately when the mobile fraction fel l below 40% of that in unperturbed membranes. Both anti-Wr(b) antibodi es suppressed invasion completely at concentrations insufficient to af fect significantly either membrane rigidity or intramembrane protein d iffusion. A univalent anti-glycophorin A (Fab) fragment, the parent an tibody of which was previously shown to inhibit invasion strongly, had only a modest effect on invasion and induced a correspondingly small change in the mobile fraction of band 3. We conjecture that inhibition of migration of intramembrane proteins may oppose invasion by prevent ing formation of a bare zone in the host cell membrane, but antibodies against the Wr(b) determinant prevent invasion by an additional and o verriding mechanism, related perhaps to transmission of a structural t ransmembrane signal, which could uncouple the reciprocal movement of i ntramembrane proteins and the spectrin network, or to formation of ban d 3-glycophorin A-antibody clusters.