KRTCAP2

KRTCAP2 (KCP2) is a small, non-catalytic integral membrane subunit of the mammalian STT3A-containing oligosaccharyltransferase (OST) complex that supports co-translational N-glycosylation in the endoplasmic reticulum (PMID:15835887, PMID:22266900). It is an integral membrane protein with three transmembrane spans generated by alternative translation initiation, and it is retained in the ER via a functional KKxx retrieval signal at its cytosolic C-terminus, assembling into a ~500 kDa complex built around the catalytic STT3A isoform with which it interacts robustly (PMID:22266900). Rather than contributing to OST catalysis, KCP2 together with DC2 physically bridges the STT3A-OST complex to the Sec61 translocation channel, and KCP2-deficient complexes remain stable and enzymatically active (PMID:28860277); consistent with this dedicated role, its loss produces substrate-specific hypoglycosylation and accumulation of an STT3A-containing subcomplex rather than the global OST destabilization seen when core subunits are depleted (PMID:22467853). KCP2 also modulates γ-secretase activity and APP processing, with depletion causing accumulation of the C99/C83 fragments and reduced Aβ production and overexpression stimulating active γ-secretase and Aβ generation (PMID:21768116). In glioma cells, KRTCAP2 promotes proliferation, migration, and invasion and is associated with tumor-associated macrophage infiltration and an immunosuppressive microenvironment (PMID:41676149).

1. 2005 Medium

   Established that an uncharacterized 14 kDa protein physically associates with the translation/translocation machinery, raising the possibility of a new OST subunit.

   Evidence Blue Native/SDS-PAGE and mass spectrometry co-purification from ribosome-translocon complexes

   PMID:15835887

   Open questions at the time

   • No functional role assigned
   • Membership in OST not yet confirmed by orthogonal methods
2. 2011 Medium

   Linked KCP2 to a specific cellular pathway by showing it modulates APP processing and γ-secretase activity, indicating substrate-specific regulatory influence.

   Evidence siRNA knockdown, overexpression, and Western analysis of APP intermediates plus γ-secretase activity assays

   PMID:21768116

   Open questions at the time

   • Direct physical interaction with γ-secretase components not resolved
   • Mechanistic link to its OST/glycosylation function not established
3. 2012 High

   Defined KCP2 topology, ER localization, and complex membership, establishing it as a bona fide subunit of the STT3A-containing OST.

   Evidence Alternative initiation mapping, immunofluorescence, native gel analysis, and co-immunoprecipitation

   PMID:22266900

   Open questions at the time

   • Functional contribution to glycosylation not yet defined
   • Basis for STT3A preference unknown
4. 2012 High

   Distinguished KCP2 from core OST subunits by showing its loss causes substrate-specific hypoglycosylation and a stable subcomplex rather than global complex collapse.

   Evidence siRNA knockdown with glycosylation assays, native gel electrophoresis, and subunit-specific depletion comparison

   PMID:22467853

   Open questions at the time

   • Identity of the affected substrate set not fully defined
   • Mechanism of selectivity unresolved
5. 2017 High

   Resolved the functional role by demonstrating that KCP2 and DC2 tether the STT3A-OST to the Sec61 channel and are required for translocon association, not catalysis.

   Evidence CRISPR-edited human cells, biochemical fractionation, Co-IP, deletion mutagenesis, and N-glycosylation assays

   PMID:28860277

   Open questions at the time

   • Structural details of the OST-Sec61 bridge not defined
   • Relative contributions of KCP2 versus DC2 not dissected
6. 2017 Medium

   Placed KCP2 within the broader A-type OST interactome by identifying SelT as a partner whose loss depletes KCP2 and impairs N-glycosylation.

   Evidence Split-ubiquitin yeast two-hybrid screen, Co-IP, and SelT siRNA knockdown with glycosylation readout

   PMID:28928140

   Open questions at the time

   • Direct KCP2-SelT contact within the complex not mapped
   • KCP2 was a secondary subject of the study
7. 2013 Low

   Reported a candidate transient interaction with the Na+/Mg2+ exchanger SLC41A1, hinting at a possible role in client maturation.

   Evidence Split-ubiquitin yeast two-hybrid screen (not confirmed by mass spectrometry)

   PMID:23823179

   Open questions at the time

   • Single yeast two-hybrid screen not confirmed by orthogonal method
   • Proposed role in folding/maturation untested functionally
8. 2026 Medium

   Extended KRTCAP2 to disease by showing it drives glioma malignant behaviors and associates with an immunosuppressive macrophage-rich microenvironment.

   Evidence Knockdown/overexpression in glioma cell lines with proliferation/migration/invasion assays and multiplex IHC for macrophage co-localization

   PMID:41676149

   Open questions at the time

   • Mechanistic link between OST/glycosylation function and tumor phenotype not established
   • Macrophage association is correlative; causal signaling pathway undefined

• How KCP2's molecular role in STT3A-OST-Sec61 coupling mechanistically connects to its effects on γ-secretase/APP processing and to glioma progression remains unresolved.
• No causal pathway linking glycosylation function to tumor phenotype
• Substrate specificity determinants unknown
• No structural model of the OST-Sec61 bridge