Truncation of MSP1 Produces an Egress Defect. (C) RAP treatment produces a loss of mAb X509 reactivity and a shift in the IFA pattern of MSP1 to one typical of PV proteins, consistent with the predicted truncation. Numbers of DAPI-stained nuclei did not differ between control and RAP treated schizonts (mean values: 21.2 ± 3.4 and 20.6 ± 4.0 nuclei per schizont, respectively, n = 24). (D) IFA showing co-localization of truncated MSP1 with SERA5 indicating a PV location. The punctate localization of SUB1 and the microneme protein AMA1 indicates normal organelle biogenesis. (E) IFA showing lack of surface-bound MSP1 on merozoites of RAP-treated 3D7MSP1flox42C1 clone E3. Antibodies to AMA1 (which is expressed on free merozoites) were used as a control. Analysis of RAP-treated 3D7MSP1flox42C parasites showed highly efficient excision, resulting in exclusive expression of truncated MSP1 in mature schizonts at the end of the same erythrocytic cycle (C). No effects on merozoite development were discernible. The modified MSP1 was trafficked to the PV as expected for a non-membrane bound merozoite surface protein (C–D) but was not present on the surface of free merozoites (E)

Das S, Hertrich N, Perrin AJ, Withers-Martinez C, Collins CR, Jones ML, Watermeyer JM, Fobes ET, Martin SR, Saibil HR, Wright GJ, Treeck M, Epp C, Blackman MJ. Processing of Plasmodium falciparum Merozoite Surface Protein MSP1 Activates a Spectrin-Binding Function Enabling Parasite Egress from RBCs. Cell Host Microbe. 2015 18(4):433-44. PMID: 26468747