A Microtubule-Associated Protein Is Essential for Malaria Parasite Transmission.

Mature gametocytes of Plasmodium falciparum display a banana (falciform) shape conferred by a complex array of subpellicular microtubules (SPMT) associated with the inner membrane complex (IMC). Microtubule-associated proteins (MAPs) define MT populations and modulate interaction with pellicular components. Several MAPs have been identified in Toxoplasma gondii, and homologues can be found in the genomes of Plasmodium species, but the function of these proteins for asexual and sexual development of malaria parasites is still unknown. Here, we identified a novel subpellicular MAP, termed SPM3, that is conserved within the genus Plasmodium, especially within the subgenus Laverania, but absent in other Apicomplexa. Conditional knockdown and targeted gene disruption of Pfspm3 in Plasmodium falciparum cause severe morphological defects during gametocytogenesis, leading to round, nonfalciform gametocytes with an aberrant SPMT pattern. In contrast, Pbspm3 knockout in Plasmodium berghei, a species with round gametocytes, caused no defect in gametocytogenesis, but sporozoites displayed an aberrant motility and a dramatic defect in invasion of salivary glands, leading to a decreased efficiency in transmission. Electron microscopy revealed a dissociation of the SPMT from the IMC in Pbspm3 knockout parasites, suggesting a function of SPM3 in anchoring MTs to the IMC. Overall, our results highlight SPM3 as a pellicular component with essential functions for malaria parasite transmission. IMPORTANCE A key structural feature driving the transition between different life cycle stages of the malaria parasite is the unique three-membrane pellicle, consisting of the parasite plasma membrane (PPM) and a double membrane structure underlying the PPM termed the inner membrane complex (IMC). Additionally, there are numerous linearly arranged intramembranous particles (IMPs) linked to the IMC, which likely link the IMC to the subpellicular microtubule cytoskeleton. Here, we identified, localized, and characterized a novel subpellicular microtubule-associated protein unique to the genus Plasmodium. The knockout of this protein in the human-pathogenic species P. falciparum resulted in malformed gametocytes and aberrant microtubules. We confirmed the microtubule association in the P. berghei rodent malaria homologue and show that its knockout results in a perturbed microtubule architecture, aberrant sporozoite motility, and decreased transmission efficiency.

Characterization of Apicomplexan Amino Acid Transporters (ApiATs) in the Malaria Parasite Plasmodium falciparum.

During the symptomatic human blood phase, malaria parasites replicate within red blood cells. Parasite proliferation relies on the uptake of nutrients, such as amino acids, from the host cell and blood plasma, requiring transport across multiple membranes. Amino acids are delivered to the parasite through the parasite-surrounding vacuolar compartment by specialized nutrient-permeable channels of the erythrocyte membrane and the parasitophorous vacuole membrane (PVM). However, further transport of amino acids across the parasite plasma membrane (PPM) is currently not well characterized. In this study, we focused on a family of Apicomplexan amino acid transporters (ApiATs) that comprises five members in Plasmodium falciparum. First, we localized four of the P. falciparum ApiATs (PfApiATs) at the PPM using endogenous green fluorescent protein (GFP) tagging. Next, we applied reverse genetic approaches to probe into their essentiality during asexual replication and gametocytogenesis. Upon inducible knockdown and targeted gene disruption, a reduced asexual parasite proliferation was detected for PfApiAT2 and PfApiAT4. Functional inactivation of individual PfApiATs targeted in this study had no effect on gametocyte development. Our data suggest that individual PfApiATs are partially redundant during asexual in vitro proliferation and fully redundant during gametocytogenesis of P. falciparum parasites. IMPORTANCE Malaria parasites live and multiply inside cells. To facilitate their extremely fast intracellular proliferation, they hijack and transform their host cells. This also requires the active uptake of nutrients, such as amino acids, from the host cell and the surrounding environment through various membranes that are the consequence of the parasite's intracellular lifestyle. In this paper, we focus on a family of putative amino acid transporters termed ApiAT. We show expression and localization of four transporters in the parasite plasma membrane of Plasmodium falciparum-infected erythrocytes that represent one interface of the pathogen to its host cell. We probed into the impact of functional inactivation of individual transporters on parasite growth in asexual and sexual blood stages of P. falciparum and reveal that only two of them show a modest but significant reduction in parasite proliferation but no impact on gametocytogenesis, pointing toward dispensability within this transporter family.