APEX-based proximity labeling in Plasmodium identifies a membrane protein with dual functions during mosquito infection

Transmission of the malaria parasite Plasmodium to mosquitoes necessitates gamete egress from red blood cells to allow zygote formation and ookinete motility to enable penetration of the midgut epithelium. Both processes are dependent on the secretion of proteins from distinct sets of specialized vesicles. Inhibiting some of these proteins has shown potential for blocking parasite transmission to the mosquito. To identify new transmission blocking vaccine candidates, we defined the microneme content from ookinetes of the rodent model organism Plasmodium berghei using APEX2-mediated rapid proximity-dependent biotinylation. Besides known proteins of ookinete micronemes, this identified over 50 novel candidates and sharpened the list of a previous survey based on subcellular fractionation. Functional analysis of a first candidate uncovered a dual role for this membrane protein in male gametogenesis and ookinete midgut traversal. Mutation of a putative trafficking motif in the C-terminus led to its mis-localization in ookinetes and affected ookinete to oocyst transition but not gamete formation. This suggests the existence of distinct functional and transport requirements for Plasmodium proteins in different parasite stages. SignificanceThe genome of the malaria parasite Plasmodium contains over 5500 genes, of which over 30% have no assigned function. Transmission of Plasmodium spp. to the mosquito contains several essential steps that can be inhibited by antibodies or chemical compounds. Yet few proteins involved in these processes are characterized, thus limiting our capacity to generate transmission interfering tools. Here, we establish a method to rapidly identify proteins in a specific compartment within the parasite that is essential for establishment of an infection within the mosquito, and identify over 50 novel candidate proteins. Functional analysis of the top candidate identifies a protein with two independent essential functions in subsequent steps along the Plasmodium life cycle within the mosquito. HighlightsO_LIfirst use of APEX based proximity ligation in Apicomplexa C_LIO_LIidentification of >50 putative ookinete surface proteins C_LIO_LInovel membrane protein essential for microgamete egress and ookinete migration C_LIO_LIputative trafficking motif essential in ookinetes but not gametes C_LI