Actins have prominent roles in the malaria parasite and two actin isoforms, which are both divergent from actin of higher eukaryotes and from each other, are present. Actin I is ubiquitously found in all stages of the parasite, and has an important role in cell motility and invasion. In vitro studies have revealed that actin I forms short and unstable filaments which have been difficult to visualize in vivo. We developed an antibody that recognizes actin in the motile ookinete in filament-like structures and in the gametocyte as a constituent of the cytoskeleton. Post-translational modifications (PTM) change the properties of proteins and it is known that actin in other eukaryotic cells is extensively modified. We have therefore carried out proteomics analysis of P. berghei actin I with the aim to identify PTMs comparing three stages of the parasite. Our analysis shows that actin I is modified at many amino acids and that there are stage-specific modifications.

We have extensively studied actin II. This protein is expressed only in the mosquito stages and employing reverse genetics approaches two functions are revealed. It is needed for male gametogenesis and later on for the development of the sporogonic oocyst, the stage at which infectious sporozoites are formed in the mosquito. The function at the molecular level is not yet completely understood for either of the two actins but our studies have revealed that the functions of the two actins are not equivalent in vivo. Through gene replacement studies we have determined that the function of actin II during gametogenesis is dependent upon the ability of the protein to form long and stable filaments while this is not sufficient for oocyst development. We have also shown that gene regulation is different in these two stages. Site-specific mutagenesis has revealed the importance of a specific PTM in actin II function. Our present work aims to further understand the function of actin II through identification of PTMs and interacting proteins.