TSN

TSN (PF3D7_1136300) is a multifunctional nuclear protein of P. falciparum that couples intrinsic nuclease activity to RNA metabolism and spliceosome assembly (PMID:18035357, PMID:20713134). It preferentially cleaves single-stranded RNA and binds RNA through its Tudor domain, with both activities sensitive to the micrococcal nuclease inhibitor pdTp, indicating a staphylococcal-nuclease-type catalytic mechanism (PMID:18035357). TSN engages the parasite nuclear machinery through domain-partitioned interactions: its SN domains bind transcriptional and other nuclear regulators including SPT2, a cold-shock DNA-binding protein, and a ubiquitin-conjugating enzyme, while its Tudor domain mediates a methylation-dependent interaction with the spliceosomal Sm protein PfSmD1, linking TSN to spliceosome assembly (PMID:20713134, PMID:24047207). TSN function is essential at the trophozoite stage, as both siRNA silencing and pdTp inhibition block parasite development at this point (PMID:18486605). During apoptosis-like cell death, TSN is a substrate of the parasite metacaspase PfMCA-2, which cleaves it at multiple sites through a non-canonical mechanism requiring the non-conserved Tyr224 (together with catalytic Cys157) of PfMCA-2; this proteolysis of TSN and other spliceosomal components impairs mRNA splicing and contributes to parasite death (PMID:26136076, PMID:29317266, PMID:35670460).

1. 2007 High

   Establishing whether the parasite TSN was catalytically active resolved that it is a bona fide ssRNA nuclease whose RNA-binding function maps to the Tudor domain, defining its core biochemical identity.

   Evidence In vitro nuclease/RNA-binding assays with pdTp inhibition and immunolocalization in P. falciparum

   PMID:18035357

   Open questions at the time

   • Physiological RNA substrates in the cell not identified
   • Relative contribution of the individual SN domains to catalysis not resolved
2. 2008 Medium

   Loss-of-function tests addressed whether TSN activity is required for the parasite, showing it is essential specifically at the trophozoite stage.

   Evidence siRNA knockdown plus pdTp chemical inhibition with growth and morphological readouts in P. falciparum

   PMID:18486605

   Open questions at the time

   • Molecular cause of the trophozoite-stage arrest not defined
   • pdTp is not TSN-specific, so off-target contribution to phenotype not excluded
3. 2010 Medium

   Interaction screening addressed which nuclear partners TSN engages and through which domains, revealing SN-domain interactions with transcriptional/nuclear regulators and a Tudor-domain link to the spliceosomal Sm protein PfSmD1.

   Evidence Bacterial and yeast two-hybrid screens with domain mapping and partner immunolocalization in P. falciparum

   PMID:20713134

   Open questions at the time

   • Interactions largely rest on two-hybrid data with limited in vitro validation
   • Functional consequence of each interaction not tested
4. 2013 Medium

   Characterizing the Tudor domain–PfSmD1 interaction determined that it is methylation-dependent, mechanistically connecting TSN to spliceosome assembly.

   Evidence Yeast two-hybrid, in vitro pull-down, and immunofluorescence co-localization in P. falciparum

   PMID:24047207

   Open questions at the time

   • The methyltransferase generating the PfSmD1 methyl mark not identified
   • Direct role of this interaction in splicing not functionally demonstrated
5. 2015 Medium

   Proteomic analysis of stress-induced death addressed the fate of TSN during apoptosis-like death, showing it is degraded by VAD-FMK-binding proteases with concurrent splicing impairment.

   Evidence Quantitative global proteomics with mRNA splicing readouts under apoptosis-inducing stress in P. falciparum

   PMID:26136076

   Open questions at the time

   • Correlative link between TSN degradation and splicing defect not causally isolated
   • Specific protease responsible not identified in this study
6. 2018 Medium

   Reconstitution identified the responsible protease, establishing TSN as a direct natural substrate of the parasite metacaspase PfMCA-2.

   Evidence In vitro cleavage of recombinant TSN by PfMCA-2 with Z-FA-FMK inhibition and western blot detection in P. falciparum

   PMID:29317266

   Open questions at the time

   • Cleavage sites within TSN not mapped
   • Catalytic residues of PfMCA-2 required for TSN cleavage not yet defined
7. 2021 High

   Active-site mutagenesis dissected how PfMCA-2 recognizes TSN, showing a non-canonical mechanism in which Tyr224 (with catalytic Cys157) is required specifically for TSN engagement and cleavage.

   Evidence In vitro and in vivo cleavage assays with PfMCA-2 mutants (Cys157A, His205A, Tyr224A) and an artificial z-GRR-AMC substrate in P. falciparum

   PMID:35670460

   Open questions at the time

   • Cleaved TSN fragments' downstream fate and effect on nuclease/splicing function not resolved
   • Structural basis of the Tyr224-dependent TSN recognition not determined

• How TSN's nuclease and spliceosome-assembly functions are integrated to sustain the essential trophozoite-stage role, and how their disruption by PfMCA-2 cleavage executes parasite death, remains unresolved.
• In vivo RNA substrates of TSN nuclease activity unknown
• Mechanistic connection between TSN loss and trophozoite arrest undefined
• Whether PfMCA-2 cleavage inactivates the splicing versus nuclease function specifically is untested