SPCS3

SPCS3 (SPC22/23) is an essential, non-catalytic subunit of the endoplasmic reticulum signal peptidase complex required for the cleavage of signal peptides from secretory protein precursors (PMID:9148930, PMID:9148931). In the yeast complex, the SPCS3 ortholog Spc3p assembles with the catalytic Sec11p and the non-essential Spc1p and Spc2p; unlike the latter two, Spc3p is indispensable for cell viability and for signal peptidase activity, establishing it as a second essential subunit beyond the catalytic core (PMID:9148931, PMID:9148930). Depletion of the gene causes accumulation of uncleaved secretory precursors in vivo and abolishes signal peptidase activity in vitro (PMID:9148930, PMID:9148931). Independent of its housekeeping role in protein maturation, SPCS3 acts as a host restriction factor against chikungunya virus, binding the viral glycoprotein E1 to limit virion production in macrophages; the positively selected E1 residue V220 mediates this interaction, and SPCS3 associates with eIF3k in the cytoplasm during infection (PMID:39261662). The mouse gene is the parent of the retrotransposed paralogs Arxes1 and Arxes2, and unlike those copies is dispensable for adipogenesis (PMID:21177646).

1. 1997 High

   Established that SPCS3's ortholog is not merely an accessory factor but is functionally required for signal peptide cleavage, answering whether the catalytic Sec11p suffices for signal peptidase activity.

   Evidence Genetic depletion of SPC3 in yeast with in vivo precursor accumulation assay and in vitro signal peptidase activity assay; independent purification, complementation and reconstitution of the four-subunit complex

   PMID:9148930 PMID:9148931

   Open questions at the time

   • Does not resolve the structural basis by which SPCS3 contributes to catalysis or substrate presentation
   • Mammalian SPCS3 function inferred from yeast Spc3p rather than directly tested in the 1997 work
   • Whether SPCS3 has activities outside the signal peptidase complex was not addressed
2. 2010 Medium

   Distinguished SPCS3 from its retrotransposed paralogs by showing the parent gene is dispensable for adipogenesis, defining a functional divergence between Spcs3 and Arxes1/Arxes2.

   Evidence RNAi knockdown in adipocyte cell models with gene expression readouts and retrotransposition/phylogenetic analysis

   PMID:21177646

   Open questions at the time

   • Single-lab dispensability finding without genetic knockout confirmation
   • Does not address whether SPCS3 contributes to other differentiation programs
   • Mechanistic basis of paralog functional divergence not defined
3. 2024 Medium

   Revealed a non-canonical antiviral role: SPCS3 restricts chikungunya virus by directly engaging the viral E1 glycoprotein, identifying a host-pathogen interface shaped by positive selection.

   Evidence Co-IP/MS proteomic interaction analysis, functional validation with viral chimeras and E1 point mutants, and evolutionary selection analysis in macrophages

   PMID:39261662

   Open questions at the time

   • Single-lab finding; reciprocal validation and structural definition of the SPCS3–E1 interface not established
   • Functional role of the SPCS3–eIF3k cytoplasmic association during infection unresolved
   • Whether antiviral restriction depends on signal peptidase activity is not determined

• How SPCS3 mechanistically reconciles its essential role in signal peptide cleavage with its restriction of chikungunya virus, and whether the two functions share a structural or catalytic basis, remains open.
• No structural model of human SPCS3 within the signal peptidase complex in the corpus
• Mechanism by which SPCS3 limits virion production is uncharacterized
• Direct experimental characterization of mammalian SPCS3 catalytic contribution is absent