SUB1-ProM

SUB1-ProM (PF3D7_0507400) is a stand-alone subtilisin propeptide-like protein of blood-stage Plasmodium falciparum that functions as a selective endogenous inhibitor of the egress protease SUB1 (PMID:31942933). Its core fold, predicted by template-based modelling and ab initio structure prediction, most closely resembles the X-ray crystal structure of the SUB1 propeptide, and recombinant SUB1-ProM acts as a fast-binding, potent inhibitor of P. falciparum SUB1 while sparing SUB2 and other proteases tested, establishing selectivity for SUB1 (PMID:31942933). Consistent with this inhibitory role, SUB1-ProM localizes to the parasitophorous vacuole, the same compartment as its target protease (PMID:31942933). Beyond this in vitro inhibitory activity and localization, the physiological consequence of SUB1-ProM function during parasite egress has not been characterized in the available corpus.

1. 2020 High

   Whether PF3D7_0507400 encodes a functional protein rather than a genomic curiosity was unknown; showing it folds like a subtilisin propeptide and selectively inhibits SUB1 established it as a bona fide endogenous protease inhibitor.

   Evidence Template-based and ab initio structure prediction plus in vitro inhibition assay with recombinant protein and selectivity profiling against SUB2 and other proteases

   PMID:31942933

   Open questions at the time

   • No experimental structure of SUB1-ProM or of the SUB1-ProM:SUB1 complex
   • Inhibitory activity shown only in vitro; effect on SUB1 activity or egress in living parasites not demonstrated
   • Kinetic mechanism and binding stoichiometry not fully resolved
2. 2020 Medium

   For an inhibitor to regulate SUB1 physiologically it must reach the same compartment; localizing SUB1-ProM to the parasitophorous vacuole placed it alongside its target protease.

   Evidence Mass spectrometry and immunofluorescence microscopy in blood-stage P. falciparum

   PMID:31942933

   Open questions at the time

   • Co-localization or physical interaction with SUB1 in situ not directly shown
   • No functional consequence of PV localization demonstrated
   • Timing of expression relative to the egress program not established
3. 2020 Low

   Whether the gene bears an evolutionary relationship to the subtilase machinery it inhibits was addressed by mapping its genomic context, linking it to the subtilisin family.

   Evidence Comparative genomics / synteny analysis showing the locus adjacent to three of the four Plasmodium subtilisin-like genes

   PMID:31942933

   Open questions at the time

   • Computational synteny only; no functional experiment ties the genomic arrangement to co-regulation or function
   • Conservation across other Plasmodium species not experimentally tested

• Whether SUB1-ProM regulates SUB1 activity in vivo and is required for or modulates parasite egress remains unresolved.
• No genetic knockout or knockdown phenotype reported
• No demonstration that SUB1-ProM inhibits SUB1 within the parasitophorous vacuole in living parasites
• Regulation of SUB1-ProM expression and its temporal relationship to egress unknown