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The bud emergence ( BEM)46 proteins are evolutionarily conserved members of the α/β-hydrolase superfamily, which includes enzymes with diverse functions and a wide range of substrates. Here, we identified a P lasmodium BEM46-like protein ( PBLP) and characterized it throughout the life cycle of the rodent malaria parasite P lasmodium yoelii. The P lasmodium BEM46-like protein is shown to be closely associated with the parasite plasma membrane of asexual erythrocytic stage schizonts and exo-erythrocytic schizonts; however, PBLP localizes to unique intracellular structures in sporozoites. Generation and analysis of P . yoelii knockout (Δ pblp) parasite lines showed that PBLP has an important role in erythrocytic stage merozoite development with Δ pblp parasites forming fewer merozoites during schizogony, which results in decreased parasitemia when compared with wild-type ( WT) parasites. Δ pblp parasites showed no defects in gametogenesis or transmission to mosquitoes; however, because they formed fewer oocysts there was a reduction in the number of developed sporozoites in infected mosquitoes when compared with WT. Although Δ pblp sporozoites showed no apparent defect in mosquito salivary gland infection, they showed decreased infectivity in hepatocytes in vitro. Similarly, mice infected with Δ pblp sporozoites exhibited a delay in the onset of blood-stage patency, which is likely caused by reduced sporozoite infectivity and a discernible delay in exo-erythrocytic merozoite formation . These data are consistent with the model that PBLP has an important role in parasite invasive-stage morphogenesis throughout the parasite life cycle. [ABSTRACT FROM AUTHOR]