GEXP15

GEXP15 (PF3D7_1031600) is a Plasmodium nuclear protein that functions as a regulator of Protein Phosphatase type 1 (PP1) and is essential for parasite proliferation and transmission (PMID:31348803). It binds PP1 directly through an N-terminal RVxF docking motif and enhances the phosphatase's catalytic activity, an effect abolished by mutation of the RVxF motif; this PP1-regulatory interaction is conserved between P. berghei and P. falciparum (PMID:31348803, PMID:37628837). Beyond PP1, GEXP15 associates with larger complexes: in P. berghei it is recovered alongside PP1 in schizont- and gametocyte-stage complexes that include spliceosome and proteasome components (PMID:31348803), and in P. falciparum its GYF-like domain mediates binding to ribosomal and RNA-binding proteins (PMID:37628837). In P. falciparum GEXP15 is most highly expressed in schizonts and localizes predominantly to the nucleus (PMID:37628837), while in the early gametocyte proteome it is detected as an exported protein in the erythrocyte cytoplasm with N-terminal PEXEL/HT processing and N-acetylation (PMID:20332084). Genetic ablation in P. berghei prevents lethal and cerebral malaria infection and blocks oocyst and sporozoite formation despite gametocyte production, and phosphoproteomic profiling of knockout schizonts shows altered abundance and phosphorylation of proteins involved in gene expression regulation and invasion (PMID:31348803), consistent with GEXP15 acting as a PP1 regulatory partner controlling parasite phosphosignaling.

1. 2010 Medium

   Establishing GEXP15 as a stage-regulated, exported protein first placed it in the biology of early sexual differentiation, before any molecular activity was known.

   Evidence Quantitative label-free comparative proteomics and N-terminal peptide detection in purified P. falciparum early gametocytes

   PMID:20332084

   Open questions at the time

   • Functional consequence of PEXEL/HT N-terminal processing and N-acetylation not tested
   • Export mechanism and destination within the erythrocyte not resolved
   • No link yet to a molecular activity
2. 2019 High

   Identifying GEXP15 as an RVxF-motif PP1 partner that boosts phosphatase activity defined its core molecular function as a PP1 regulatory protein.

   Evidence In vitro pull-down, reciprocal Co-IP/MS, and in vitro phosphatase activity assays with wild-type and RVxF-mutant GEXP15 in P. berghei

   PMID:31348803

   Open questions at the time

   • Substrates whose dephosphorylation is controlled by the GEXP15–PP1 complex not directly identified
   • Structural basis of activity enhancement not resolved
   • Whether GEXP15 targets PP1 to specific substrates or subcellular sites unknown
3. 2019 Medium

   Demonstrating that GEXP15 co-purifies with PP1 in spliceosome- and proteasome-containing complexes placed the phosphatase regulator within larger machinery across asexual and sexual stages.

   Evidence Immunoprecipitation of tagged GEXP15 or PP1 followed by mass spectrometry in P. berghei schizonts and gametocytes

   PMID:31348803

   Open questions at the time

   • Direct versus indirect nature of spliceosome/proteasome associations not distinguished
   • Functional role of GEXP15 within these complexes not tested
   • Stage-specific composition of the complexes not fully resolved
4. 2019 High

   Genetic knockout established that GEXP15 is essential for both asexual blood-stage proliferation and mosquito-stage transmission, moving it from an associated factor to a required regulator.

   Evidence GEXP15 knockout in P. berghei with in vivo infection assays in BALB/c and C57BL/6 mice and mosquito feeding assays; global proteomics/phosphoproteomics of KO schizonts

   PMID:31348803

   Open questions at the time

   • Causal chain from GEXP15 loss to the transmission block not defined
   • Which altered phosphorylation events drive the phenotype not established
   • Essentiality not tested in P. falciparum
5. 2023 High

   Confirming the RVxF-dependent PP1 interaction in P. falciparum and mapping a GYF-domain link to ribosomal/RNA-binding proteins extended the PP1-regulator model to the human pathogen and identified a second interaction interface.

   Evidence In vitro pull-down and phosphatase assays with recombinant proteins, Co-IP/MS, GYF-domain pull-down, live imaging, and in silico domain analysis in P. falciparum

   PMID:37628837

   Open questions at the time

   • Functional consequence of GEXP15 binding ribosomal/RNA-binding proteins not tested
   • Domain boundaries and the conserved domain of unknown function not experimentally validated
   • How nuclear localization reconciles with the earlier exported/erythrocyte detection not addressed

• The physiological substrates and biological processes controlled by the GEXP15–PP1 complex, and how its ribosomal/RNA-binding and nuclear activities integrate with phosphatase regulation, remain unresolved.
• No direct PP1 substrate assigned to GEXP15 regulation
• Mechanistic role of the GYF-domain ribosomal interaction unknown
• No structural model of the GEXP15–PP1 complex