PRMT1

PRMT1 in Toxoplasma gondii is a cytoplasmic arginine methyltransferase that governs both RNA silencing and cell-division fidelity, and it constitutes the major contributor to arginine monomethylation in this organism (PMID:28143887). In the RNA silencing pathway, TgPRMT1 methylates the N-terminal RGG-repeat domain of the single Argonaute protein TgAgo, and this modification is required to recruit Tudor staphylococcal nuclease into the Ago complex, enabling Ca2+-dependent, guide RNA-directed target RNA cleavage (PMID:22309152). Independently, TgPRMT1 is enriched at the pericentriolar material and is essential for accurate centrosome segregation and synchronous endodyogeny: its deletion perturbs centrosome stoichiometry, produces abnormal daughter buds, and slows tachyzoite growth, defects reversed by complementation (PMID:26838719). Its broader substrate repertoire spans RNA-binding proteins, ApiAP2 transcription factors, and kinases, with a strong overlap between monomethylarginine and phosphorylation marks pointing to crosstalk between the two modifications (PMID:28143887).

1. 2012 High

   It was unknown how the Toxoplasma RNA silencing machinery assembles its effector nuclease; this work established that PRMT1-catalyzed methylation of the TgAgo RGG domain is the molecular switch that recruits Tudor staphylococcal nuclease and licenses target RNA cleavage.

   Evidence reciprocal Co-IP of the TgAgo complex, RGG-domain mutagenesis, in vitro RNA cleavage assays, and PRMT1 knockout showing loss of TSN recruitment

   PMID:22309152

   Open questions at the time

   • the biological RNA targets and physiological role of the cleavage activity are not defined
   • direct in vitro reconstitution of PRMT1 methyltransferase activity on TgAgo not shown
2. 2016 High

   Whether PRMT1 has functions beyond RNA silencing was unresolved; genetic deletion revealed an essential role in centrosome segregation and synchronous endodyogeny, localizing the enzyme to the pericentriolar material.

   Evidence Δprmt1 knockout with PRMT1mRFP complementation rescue, live-imaging localization, and microscopic analysis of centrosome stoichiometry and daughter-bud morphology

   PMID:26838719

   Open questions at the time

   • the pericentriolar substrate(s) whose methylation drives centrosome fidelity are not identified
   • mechanistic link between methyltransferase activity and centrosome segregation not established
3. 2017 Medium

   The full substrate scope of PRMT1 was unknown; comparative monomethylarginine proteomics established it as the major arginine monomethyltransferase in T. gondii and identified candidate substrates linking methylation to transcription, RNA metabolism, and signaling.

   Evidence anti-MMA affinity purification with mass spectrometry comparing wild-type and PRMT1-depleted parasites

   PMID:28143887

   Open questions at the time

   • substrates identified by proteomics without in vitro reconstitution of direct methylation
   • functional consequence of the MMA-phosphorylation crosstalk not tested
   • which candidates are direct versus indirect PRMT1 targets not distinguished

• How PRMT1 activity is coordinated between its RNA-silencing and centrosome roles, and which specific methylation events drive division fidelity, remain unresolved.
• no structural model of TgPRMT1 or its substrate recognition
• regulation of PRMT1 activity and localization not characterized
• direct causal link between individual substrate methylation and each phenotype not established