Mutant/mutation
The mutant lacks expression of FT2 and expresses GFP under the control of the constitutive and strong hsp70 promoter. The construct to delete the ft2 gene contains a GFP expression cassette. In this cassette the gfp gene is under the control of the hsp70 promoter. The mutant does not contain a drug-selectable marker (SM) since the SM has been removed by applying negative selection.

Protein (function)
To date, two folate transporters, called FT1 and FT2, have been identified in the Plasmodium genome datasets. Both proteins contain two domains with high similarity to the Leishmania biopterin transporter 1 (BT1), a defining signature of BT1 transmembrane proteins (PFAM: PF03092), which belong to the major facilitator superfamily (MFS). FT1 and FT2 facilitate the transport of folate and folate precursors when expressed in Xenopus laevis oocytes and restore normal growth in Escherichia coli bacteria deficient in folate biosynthesis and salvage. Using polyclonal antisera of uncharacterized specificity, the P. falciparum orthologues of FT1 and FT2 were shown to localize to the parasite periphery, tentatively implicating them in folate salvage rather than intraparasitic folate shuttling. Using a combination of experimental genetics and quantitative live cell microscopy, we demonstrate here that P. berghei FT2 is an apicoplast-resident membrane transport protein required for Plasmodium oocyst development and the production of infective sporozoites.
 

Phenotype
Normal numbers of oocysts. However, at day 17 only 2% of them  had entered sporulation, as compared to 50% of WT oocysts. This striking defect correlated with prominent swelling of the FT2-deficient oocysts. Extremely low numbers of ft2–sporozoites (~1% of WT) associated with mosquito salivary glands. Sporozoites are not infectious to mice and in vitro hepatocyte cultures.

Additional information
To investigate the subcellular localization and temporal expression of FT2 in the rodent malaria model parasite P. berghei, we generated transgenic parasites expressing endogenous FT2 fused to an mCherry-3xMyc tag. Microscopic examination of fluorescently labelled FT2 uncovered a striking organellar localization throughout Plasmodium blood stage development. The complete lack of organelle branching in female gametocytes was indicative of the apicoplast rather than the mitochondrion. We therefore generated a second independent parasite line that, in addition to mCherry-3xMyc-tagged FT2, expresses a GFP-fluorescent apicoplast reporter (api-GFP). Live co-localization analysis revealed a striking overlap of tagged FT2 and api-GFP. mCherry fluorescence was detected in all stages of the P. berghei life cycle, together suggesting that FT2 is a broadly expressed membrane transport protein of the apicoplast. We also  detected a small fraction of FT2 at the apical prominence of merozoites, as observed by  fluorescence and differential interference contrast imaging. Similarly, in sporulating oocysts, FT2 was detected in the branching and dividing apicoplast and in additional fluorescent foci. Inspection of sporozoites revealed that extra-plastidial FT2 localizes to an elongated structure at one end of the parasite. Using time resolved live fluorescence imaging of gliding sporozoites, this fraction was consistently detected at the apical parasite pole. Together, our analysis uncovered a striking organellar localization of P. berghei FT2 to the apicoplast and to the apex of invasive parasite stages, with abundant expression during parasite life cycle progression. 

To test whether FT2 is also required for liver stage maturation, we rescued the sporulation defect by feeding mosquitoes on mice dually infected with green-fluorescent ft2– and red-fluorescent WT (Berred) parasites. Cross-fertilization of both parasite strains in vivo led to the formation of double-fluorescent heterozygous WT x ft2–oocysts and thus to the generation of a mixed sporozoite population with individual parasites either containing or lacking the FT2 gene. Intravenous injection of the haploid sporozoites resulted in the emergence of WT and FT2-deficient blood stages three days following injection indicating norma liver stage development of FT@-deficient parasites.

From the abstract:
FT2- deficient parasites develop inflated oocysts with unusual pulp formation consisting  of numerous single-membrane vesicles, which ultimately fuse to form large cavities. Ultrastructural analysis suggests that this defect reflects aberrant sporoblast formation caused by abnormal vesicular traffic.

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