Characterization of GEXP15 as a potential regulator of Protein Phosphatase 1 and partner of ribosomal complex in Plasmodium falciparum.

The Protein Phosphatase type 1 catalytic subunit (PP1c) (PF3D7_1414400) operates in combination with various regulatory proteins to specifically direct and control its phosphatase activity. However, little is known about this phosphatase and its regulators in the human malaria parasite, Plasmodium falciparum. To address this gap, we conducted a comprehensive investigation into structural and functional characteristics of a conserved Plasmodium-specific regulator called Gametocyte EXported Protein 15, GEXP15 (PF3D7_1031600). Through in silico analysis, we identified three significant regions of interest in GEXP15: an N-terminal region housing a PP1-interacting RVxF motif, a conserved domain whose function is unknown and a GYF-like motif that potentially facilitates specific protein-protein interactions. To further elucidate the role of GEXP15, we conducted in vitro interaction studies, which demonstrated a direct interaction between GEXP15 and PP1 via the RVxF binding motif. This interaction was found to enhance phosphatase activity of PP1. Additionally, utilizing a transgenic GEXP15-tagged line and live microscopy, we observed high expression of GEXP15 in late asexual stages of the parasite, with localization predominantly in the parasite nucleus. Immunoprecipitation assays followed by mass spectrometry analyses revealed GEXP15s interaction with ribosomal and RNA binding proteins. Furthermore, through pulldown analyses of recombinant functional domains of GEXP15 tagged with a His-tag, we confirmed its binding to the ribosomal complex via the GYF domain. Collectively, our study sheds light on the PfGEXP15-PP1-ribosome interaction, which plays a crucial role in protein translation. These findings suggest that PfGEXP15 could serve as a potential target for the development of malaria drugs.