Characterization of the Regulatory AAA-ATPase Subunit Rpt3 in Plasmodium berghei as an Activator of Protein Phosphatase 1

The 26S proteasome is the main proteolytic machinery involved in protein degradation, thereby contributing to the homeostasis and stress response of eukaryotic cells. This macromolecular complex consists of a 20S core particle assembled with one or two 19S regulatory particles. Here, we describe the <i>Plasmodium berghei</i> (Pb) proteasome AAA-ATPase regulatory subunit Rpt3 and demonstrate its binding to the Protein Phosphatase 1 catalytic subunit (PP1c), which is one of the major and essential parasite phosphatases. The PbRpt3 protein enhances the activity of PP1c both in vitro and in a <i>Xenopus</i> oocyte heterologous model. Further investigation of this model suggests that the PbRpt3-PP1c interaction may occur outside of the proteasome, and it reveals that the RVxF motifs of PbRpt3 are involved in its binding and regulatory function. Moreover, the ATP-binding capacity of PbRpt3 may also contribute to its phosphatase regulatory activity. In the parasite, reverse genetic studies suggest an essential role for PbRpt3 during erythrocytic cycle of <i>P. berghei</i>, and an interactome analysis confirmed that PbRpt3 belongs to the 19S regulatory particle of the proteasome and may interact with proteins previously shown to be involved in phospholipid binding.