Mutant/mutation
The mutant lacks expression of CPβ (capping protein beta;  UIS17, upregulated in infectious sporozoites gene 17)

Protein (function)
In the genome of Plasmodium two genes, cpα (PFE1420w) and cpβ, are present that encode two F-actin capping protein subunits. CPβ has been identified in a screen for transcripts that are upregulated in infectious sporozoites (Matuschewski et al., 2002, J. Biol. Chem.).

Phenotype
The phenotype analyses indicate a reduced transformation of mature ookinetes into the oocyst stage, possibly as a result of reduced ookinete motility and penetration of the mosquito midgut epithelium. In addition, sporozoites showed strongly reduced motility and were unable to invade the salivary gland. These results indicate a role of CPβ in the motility (forward locomotion) of ookinetes and sporozoites.

Additional information
RT-PCR analyses showed transcription of cpβ in merozoites, ookinetes and sporozoites.

To test whether the phenotype of mutant parasites during mosquito development can be reversed by inheritance of a wild type cpβ copy during sporozoite formation, cpβ(−) and wild type parasites were crossed and the mixed parasites genotyped in comparison with clonal parasites before and after mosquito transmission. In mixed inoculations the cpβ(−) genotype was recovered after life cycle completion. These findings led us to conclude that the essential function of Plasmodium CPβ is restricted to the insect vector stages, and this deficiency can be rescued by transient complementation of one wild type copy during oocyst development, where a heterozygous cell undergoes multiple rounds of replication prior to sporozoite budding.

Additional information on the protein
The actin-based microfilament system drives motile processes, such as cell motility, cytokinesis and vesicle transport in eukaryotic cells. These processes require dynamic interconversion of pools of monomeric and filamentous actin (G- and F-actin respectively), regulated by a large number of accessory proteins. Capping protein (CP) is a heterodimeric protein that is a central component of actin polymerization-driven cell motility, as it restricts growth of a subset of filaments thereby allowing fast, directed polymerization from a pool of unpolymerized actin. Malaria and related parasites, such as Toxoplasma gondii, employ their own actin/myosin motor machinery to propel themselves into the host cell. In addition, actin-based motility drives parasite locomotion and transmigration en route to the final target cell. The actin motor machinery of Plasmodium features the short tailless motor MyoA, tethered to the inner membrane complex by accessory proteins, and very short polymers of actin that are linked to thrombospondin-related anonymous protein (TRAP)/MIC2-family invasins via aldolase and possibly other proteins. This arrangement mediates gliding on the substratum, apparently by moving F-actin–receptor complexes from the apical tip backwards along the parasite's longitudinal axis. The regulation of this motor machinery remains elusive. Plasmodium and related Apicomplexa encode only a fraction of the conventional microfilament regulators, with many protein families missing entirely. Given the intrinsic instability of parasite actin polymers F-actin end-capping proteins are expected to be required for both orchestrated F-actin assembly and sustained filament stability. In Plasmodium genomes, two capping protein subunits can be identified. Protein sequences of the putative beta capping protein subunits from different Plasmodium species (PFE0880c and PBANKA_123240 for P. falciparum and P. berghei CPβ respectively) share around 25% sequence identity with those of yeast, chicken and human. These genes are among the most divergent within the family. However, the majority of the key residues stabilizing the heterodimer and those implicated in actin binding are present. The Plasmodium genomes also encode the corresponding putative capping protein alpha-subunit (CPα; f-actin capping protein alpha subunit, putative; PFE1420w; PBANKA_124310; ).
Evidence in the paper is presented that PbCP displays bona fide F-actin capping activity in vitro.

Other mutants