Among the three major infectious disease killers in the world - AIDS, tuberculosis and malaria - malaria is unique in that it requires an intermediate insect vector (the Anopheles mosquito) for transmission to occur. This absolute requirement provides novel opportunities to develop strategies to control the spread of disease. In the mosquito, a major bottleneck in parasite numbers occurs in the midgut, making the midgut stages of the parasite cycle especially vulnerable to interference. The goal of the present project is to define the molecular mechanisms operating during the Plasmodium ookinete invasion of the mosquito midgut. The aims include the following. 1) Further characterize the recently discovered plasminogen-mediated mechanism of ookinete midgut invasion. Key molecules involved in activation on the ookinete surface, of plasminogen into active plasmin will be further defined. In addition, luminal matrix components targeted by the ookinete-associated plasmin will be defined. 2) During the current project period we discovered that the 12-amino acid peptide MP2, binds tightly to the luminal surface of the mosquito midgut while blocking ookinete invasion. With the same strategy previously used for a different blocking peptide (SM1) we will identify the putative midgut receptor to which MP2 binds and identify the ookinete protein that interacts with this receptor. Characterization of this new parasite-mosquito pathway for midgut invasion might provide important new tools to interfere with malaria transmission. 3) We have previously found that the rodent malaria P. berghei can invade the mosquito midgut by more than one pathway, one that is sensitive to SM1 peptide blocking and the other that is not. We will investigate whether or not field isolates of P. falciparum and P. vivax display differential sensitivities to inhibition by the SM1 and MP2 peptides. These findings may have important implications for the implementation of transmission-blocking strategies to control malaria.