SIS1

SIS1 (PF3D7_0213100) is a cytosolic Type II J-domain cochaperone (Hsp40) of Plasmodium falciparum that functions as a partner to the major cytosolic chaperone PfHsp70-1 (PMID:25701168). Recombinant SIS1 forms dimers and stimulates the in vitro ATPase activity of PfHsp70-1, the defining biochemical activity of a functional Hsp70 cochaperone (PMID:25701168). SIS1 exhibits substrate selectivity that distinguishes it from the paralogous Type I cochaperone PfYdj1: SIS1 does not associate with the ribonucleotide reductase small subunit PfR2, whereas PfR2 recruitment to PfHsp70-1 is specific to PfYdj1 (PMID:40938117). Beyond its cytosolic localization, PfHsp70-1 stimulation, and this negative substrate result, no further mechanistic detail — including physiological client proteins, structural basis of the J-domain interaction, or an in vivo phenotype — has been characterized for SIS1 in the available corpus.

1. 2015 Medium

   Establishing whether PF3D7_0213100 functions as a bona fide Hsp70 cochaperone required demonstrating both its cellular compartment and a direct effect on Hsp70 activity, which this work provided by placing SIS1 in the cytosol and showing it stimulates PfHsp70-1 ATPase turnover.

   Evidence Subcellular fractionation/localization plus in vitro ATPase stimulation and dimerization assays on recombinant SIS1

   PMID:25701168

   Open questions at the time

   • No physiological client/substrate proteins identified
   • No structural characterization of the J-domain/PfHsp70-1 interaction
   • No in vivo essentiality or loss-of-function phenotype established
2. 2025 Medium

   Whether SIS1 shares substrates with the paralogous cochaperone PfYdj1 was resolved by testing ribonucleotide reductase subunit PfR2, showing that PfR2 binds PfHsp70-1 via the Type I cochaperone PfYdj1 but not via the Type II cochaperone SIS1, establishing cochaperone substrate selectivity.

   Evidence Co-immunoprecipitation/interaction assays contrasting PfR2 binding to PfYdj1 versus SIS1 in P. falciparum 3D7

   PMID:40938117

   Open questions at the time

   • Negative result establishes what SIS1 does not bind, but no positive SIS1-specific client was identified
   • Molecular basis for the differential selectivity between Type I and Type II cochaperones not defined

• The physiological client repertoire of SIS1 and its role in parasite biology remain undefined.
• No positive SIS1 client protein identified
• No genetic loss-of-function or essentiality data
• No structural model of SIS1 or its Hsp70 engagement