MAT1

MAT1 (PF3D7_0512300) is a nuclear effector subunit of a trimeric CDK-activating kinase-like complex in Plasmodium falciparum, assembling with the CDK7 homolog Pfmrk and the cyclin H homolog PfCyc1 (PMID:16737745, PMID:28611247). Its C-terminal domain physically engages Pfmrk and functions as the kinase-activator domain, stimulating Pfmrk activity in a cyclin-dependent manner (PMID:16737745, PMID:20332005). Functionally, MAT1 both promotes RNA polymerase II CTD phosphorylation and redirects Pfmrk substrate specificity, enabling phosphorylation of the DNA replication proteins PfRFC-5 and PfMCM6, thereby linking the complex to transcriptional regulation and DNA replication (PMID:16737745, PMID:20332005). Notably, MAT1-activated Pfmrk phosphorylates the CTD but not PfPK5, indicating the complex acts as a CTD kinase rather than a canonical CAK in this context (PMID:16737745). MAT1 and Pfmrk co-localize to the parasite nucleus (PMID:20332005). Beyond its role within this complex, no genetic loss-of-function phenotype or structural characterization of MAT1 itself has been reported in the available corpus.

1. 2006 High

   Establishing whether Pfmrk required an accessory activator resolved how this Plasmodium CDK7 homolog is switched on; MAT1 was shown to be the effector that stimulates Pfmrk kinase activity toward the RNA Pol II CTD.

   Evidence In vitro kinase assays with recombinant proteins testing RNA Pol II CTD and PfPK5 as substrates

   PMID:16737745

   Open questions at the time

   • Did not map the region of MAT1 responsible for activation
   • Physiological substrates in the parasite not defined
   • No structural basis for activation
2. 2010 Medium

   Defining the physical basis of activation and the subcellular context showed that MAT1 directly binds Pfmrk through its C-terminal domain and that the two proteins co-localize in the nucleus, consistent with an in vivo assembly.

   Evidence Reciprocal co-immunoprecipitation with domain mapping and immunofluorescence of GFP/RFP-tagged proteins

   PMID:20332005

   Open questions at the time

   • Single-lab interaction data without structural detail
   • Timing of complex assembly across the cell cycle unknown
3. 2010 Medium

   Determining whether MAT1 does more than activate kinase output revealed that it confers substrate specificity, directing Pfmrk to phosphorylate the DNA replication factors PfRFC-5 and PfMCM6.

   Evidence Bacterial two-hybrid screen of a P. falciparum cDNA library and in vitro phosphorylation assays

   PMID:20332005

   Open questions at the time

   • In vitro phosphorylation not validated as an in vivo regulatory event
   • Functional consequence for replication timing not tested
4. 2017 Medium

   Confirming the native complex composition, pulldown of the cyclin subunit demonstrated that MAT1, Pfmrk, and PfCyc1 form a trimeric complex in the parasite rather than only in reconstituted systems.

   Evidence Co-immunoprecipitation of PfCyc1 from parasite lysate with mass spectrometry/western detection

   PMID:28611247

   Open questions at the time

   • Stoichiometry and structure of the trimeric complex not resolved
   • Regulation of complex formation across the life cycle unknown

• The essentiality and in vivo function of MAT1 itself — through genetic disruption and separation of its transcriptional versus replication-linked roles — remain unresolved.
• No loss-of-function/knockout phenotype for MAT1 reported
• No structure of MAT1 or the trimeric complex
• In vivo relevance of PfRFC-5/PfMCM6 phosphorylation not established