KRTCAP2

KRTCAP2 (KCP2) is a non-catalytic subunit of the STT3A-containing oligosaccharyltransferase (OST) complex that couples co-translational N-glycosylation to the Sec61 protein translocation channel in the endoplasmic reticulum. It is a ~14 kDa integral ER membrane protein with three transmembrane spans and a C-terminal KKxx ER-retrieval signal that co-purifies with the ~500 kDa STT3A-OST complex and, together with the accessory subunit DC2, physically bridges the OST to the Sec61 translocon (PMID:15835887, PMID:22266900, PMID:28860277). Loss of KRTCAP2 does not destabilize the STT3A complex or abolish its catalytic activity but uncouples it from the translocon, resulting in substrate-selective hypoglycosylation of co-translational OST substrates (PMID:22467853, PMID:28860277). KRTCAP2 also interacts with Selenoprotein T (SelT), whose depletion destabilizes OST-A subunits and impairs N-glycosylation (PMID:28928140).

1. 2005 High

   The identity of all mammalian OST subunits was incomplete; co-purification and mass spectrometry revealed KCP2 as a novel component of the OST complex, expanding the known subunit repertoire.

   Evidence 2D Blue Native PAGE/SDS-PAGE with MS from solubilized ribosomes

   PMID:15835887

   Open questions at the time

   • Topology and membrane integration of KCP2 were unknown
   • Functional contribution of KCP2 to N-glycosylation was not tested
   • Which OST isoform (STT3A vs STT3B) KCP2 preferentially associates with was not determined
2. 2012 High

   Whether KCP2 is a peripheral or integral membrane protein, and which catalytic isoform it associates with, was resolved: KCP2 has three transmembrane spans, localizes to the ER via a KKxx retrieval signal, and preferentially assembles into the STT3A-containing ~500 kDa OST complex.

   Evidence Topology mapping, live-cell ER imaging, native gel electrophoresis, and reciprocal co-immunoprecipitation in human cells

   PMID:22266900

   Open questions at the time

   • How KCP2 contributes to OST function or substrate selection was still unknown
   • Whether KCP2 contacts the Sec61 translocon was not addressed
3. 2012 High

   The functional consequence of KCP2 loss was defined: depletion does not globally destabilize the OST complex but causes substrate-selective hypoglycosylation and accumulation of a novel STT3A subcomplex, establishing KCP2 as an accessory subunit that enhances processing of specific co-translational substrates.

   Evidence siRNA knockdown with native gel analysis and glycosylation substrate profiling across multiple substrates

   PMID:22467853

   Open questions at the time

   • The molecular mechanism of substrate selectivity was not identified
   • Whether KCP2 depletion phenocopies STT3A loss or represents a distinct, partial defect was unclear
4. 2017 High

   The key mechanistic question—how the STT3A-OST is physically coupled to the Sec61 translocon for co-translational glycosylation—was answered: DC2 and KCP2 together bridge this interaction, and cells lacking either subunit phenocopy STT3A loss for co-translational glycosylation despite retaining stable, catalytically active STT3A complexes.

   Evidence CRISPR-edited human cells lacking DC2 or KCP2, co-immunoprecipitation of OST–translocon complexes, deletion mutagenesis, and glycosylation assays

   PMID:28860277

   Open questions at the time

   • Structural basis of the KCP2–DC2–Sec61 interaction is not resolved
   • Whether KCP2 directly contacts Sec61 or acts solely through DC2 is unknown
   • Relative individual contributions of DC2 versus KCP2 to translocon coupling are not fully separated
5. 2017 Medium

   The OST-A interaction network was expanded when Selenoprotein T was identified as a KCP2 binding partner whose depletion destabilizes OST-A subunits and impairs POMC N-glycosylation, linking redox biology to OST function.

   Evidence Yeast split-ubiquitin screen, co-immunoprecipitation, siRNA knockdown, and POMC glycosylation assay

   PMID:28928140

   Open questions at the time

   • Direct binding interface between SelT and KCP2 is undefined
   • Whether SelT is a stable OST-A subunit or a transient regulatory factor is not settled
   • Findings from a single laboratory

• The structural basis by which KCP2 and DC2 jointly bridge the STT3A-OST to Sec61, the precise contact surfaces involved, and the mechanism through which KCP2 confers substrate selectivity remain unresolved.
• No high-resolution structure of the KCP2-containing OST–Sec61 supercomplex exists
• Substrate features that make glycosylation KCP2-dependent are not defined
• Physiological consequences of KCP2 loss in animal models have not been reported