SUB3

SUB3 (PF3D7_0507200) is a subtilisin-like serine protease expressed at late asexual blood stages of Plasmodium falciparum (PMID:22285468). Its mature region (PfSUB3m, Thr516–Glu769) exhibits PMSF-sensitive proteolytic activity in vitro, and substitution of its active-site serine with alanine abolishes catalysis, establishing a canonical serine-protease mechanism dependent on this residue; both the full-construct and mature forms undergo truncation to an active ~25 kDa species (PMID:22285468). In vitro, mature recombinant SUB3 cleaves recombinant P. falciparum profilin (PfPRF) in a dose-dependent manner, identifying PfPRF as a substrate and pointing toward a possible role in actin-dynamics-dependent processes (PMID:24080030). Beyond its catalytic characterization and the in vitro PfPRF cleavage, the in vivo substrates, subcellular localization, and precise placement of SUB3 within merozoite egress or invasion pathways have not been characterized in the available corpus.

1. 2004 Low

   Before any individual biochemical study, the question was whether SUB3 belonged to a functionally relevant protease family in blood-stage parasites; cataloguing it among three asexual-stage subtilisin-like proteases framed it as a candidate protease of the invasive parasite.

   Evidence Review of experimental literature on Plasmodium subtilisin-like proteases

   PMID:15225303

   Open questions at the time

   • Review attributes secretory-organelle localization to only two of three family members without specifying SUB3, so SUB3 localization is not individually established
   • No primary experiment on SUB3 reported in this source
   • No substrate or catalytic data for SUB3
2. 2012 High

   It was unknown whether SUB3 is a catalytically active enzyme and when it is expressed; recombinant expression with active-site mutagenesis showed the mature protein is a genuine serine protease whose activity depends on the catalytic serine, and it is expressed at late asexual blood stages.

   Evidence Recombinant expression in E. coli, in vitro proteolytic assay, PMSF inhibition, active-site serine-to-alanine mutagenesis, qRT-PCR, IFA, and Western blot

   PMID:22285468

   Open questions at the time

   • No physiological substrate identified in this study
   • Subcellular localization of SUB3 not resolved
   • Functional role in the parasite life cycle not tested
3. 2013 Medium

   With SUB3 established as an active protease, the open question was what it cleaves; an in vitro reconstitution identified P. falciparum profilin as a dose-dependent substrate, linking SUB3 to actin-regulatory machinery.

   Evidence In vitro proteolysis assay with recombinant mature SUB3 and recombinant PfPRF

   PMID:24080030

   Open questions at the time

   • Single-lab in vitro reconstitution with recombinant proteins; PfPRF cleavage not confirmed in vivo
   • Functional consequence of PfPRF cleavage for actin dynamics, egress, or invasion untested
   • No cleavage-site mapping or physiological timing established

• The in vivo substrate repertoire, subcellular localization, and mechanistic role of SUB3 in merozoite egress or invasion remain unresolved.
• No gene knockout/knockdown phenotype for SUB3
• PfPRF cleavage not validated in the parasite
• SUB3 native localization not individually confirmed