SEC63

SEC63 is an ER-resident transmembrane J-domain co-chaperone that partners with SEC61 and SEC62 to form the post-translational protein translocation channel, and additionally facilitates co-translational import of precursors bearing slowly gating or suboptimal signal peptides (PMID:10799540, PMID:22375059, PMID:32133789). Cryo-EM structures show that SEC63 induces partial opening of the SEC61 lateral gate through cytosolic and luminal domain contacts, while its J-domain recruits BiP/Kar2 to drive substrate translocation; CK2-mediated phosphorylation of SEC63 enhances SEC62 binding to complete channel activation (PMID:33398175, PMID:8305736, PMID:23287549). Beyond translocation, SEC63 deficiency selectively activates the IRE1α–XBP1 branch of the UPR and impairs GPS cleavage and maturation of polycystin-1, linking SEC63 to polycystic kidney and liver disease (PMID:25844898, PMID:30745418). Loss-of-function mutations in SEC63 cause autosomal dominant polycystic liver disease in humans (PMID:15133510).

1. 1993 High

   Establishing that SEC63 functionally cooperates with the ER luminal Hsp70 chaperone Kar2/BiP resolved how the J-domain of an ER translocon component couples to a chaperone motor for substrate translocation.

   Evidence Allele-specific suppression, synthetic lethality, and Northern blot in yeast sec63/kar2 mutants

   PMID:8305736

   Open questions at the time

   • Direct physical contact between SEC63 J-domain and Kar2 was inferred genetically, not shown biochemically in this study
   • Whether SEC63–Kar2 cooperation extends to co-translational substrates was not tested
2. 1993 Medium

   Identification of SEC66/HSS1 and SON1–SON5 as suppressors of sec63 alleles expanded the translocon to additional subunits and hinted at SEC63 roles beyond secretory protein import.

   Evidence High-copy and extragenic suppressor screens in yeast, translocation precursor accumulation assays

   PMID:8257794 PMID:8514125

   Open questions at the time

   • Mechanistic basis of SON1-mediated suppression of sec63 was not determined
   • Whether SEC66 is a stable stoichiometric subunit or regulatory factor was unclear
3. 2000 High

   Demonstrating that mammalian SEC63 forms a ribosome-free complex with SEC61 and SEC62 established the existence of a dedicated post-translational translocon in the mammalian ER, resolving whether yeast findings were conserved.

   Evidence Biochemical fractionation and co-immunoprecipitation from mammalian ER membranes

   PMID:10799540

   Open questions at the time

   • Substrate identity for the mammalian SEC62/SEC63 translocon was unknown
   • Structural organization of the complex was not resolved
4. 2004 High

   Identifying SEC63 loss-of-function mutations as the genetic cause of autosomal dominant polycystic liver disease linked ER translocation machinery to organ-level cystic pathology for the first time.

   Evidence Genetic mapping and mutation sequencing in affected human families

   PMID:15133510

   Open questions at the time

   • Which SEC63-dependent client protein(s) are responsible for cystogenesis was unknown
   • Whether haploinsufficiency or a two-hit mechanism drives disease was not established
5. 2012 High

   siRNA studies distinguished SEC63 from SEC62 substrate specificity: SEC63 is required for co-translational transport of specific signal-peptide precursors, not solely post-translational substrates, broadening its functional scope.

   Evidence siRNA knockdown in HeLa cells with semi-permeabilized cell translocation assays for multiple defined substrates

   PMID:22375059

   Open questions at the time

   • Signal peptide features that determine SEC63 dependence were not yet defined
   • Whether BiP/J-domain interaction is required for the co-translational role was not tested
6. 2012 Medium

   Mapping CK2-mediated phosphorylation of SEC63 at S574, S576, and S748 and showing it enhances SEC62 binding revealed a post-translational regulatory mechanism for translocon assembly.

   Evidence In vitro kinase assays, pull-down and co-immunoprecipitation in human cells

   PMID:23287549

   Open questions at the time

   • In vivo phosphorylation dynamics and whether CK2 activity is regulated under ER stress were not examined
   • Functional consequence of phosphorylation on translocation efficiency was not directly measured
7. 2015 High

   Showing that SEC63 deficiency selectively activates the IRE1α–XBP1 UPR branch and that SEC63 complexes with polycystin-1 to regulate its GPS cleavage provided a mechanistic link from the translocon to cystic disease pathogenesis.

   Evidence Conditional knockout mice, co-immunoprecipitation, PC1 GPS cleavage assay, in vivo XBP1s rescue

   PMID:25844898

   Open questions at the time

   • How SEC63 physically promotes GPS cleavage of PC1 was not resolved
   • Whether the SEC63–PC1 interaction is direct or mediated by the translocon complex was not determined
8. 2019 High

   Demonstrating that combined loss of SEC63 and IRE1α/XBP1 in collecting duct causes kidney injury rescuable by XBP1s established that basal UPR compensates for SEC63 loss to maintain renal proteostasis.

   Evidence Collecting duct–specific conditional knockout mice with functional and histological phenotyping, XBP1s re-expression rescue

   PMID:30745418

   Open questions at the time

   • Identity of the critical XBP1 target genes that compensate for SEC63 loss was not defined
9. 2020 High

   Unbiased proteomic identification of SEC62/SEC63-dependent substrates and signal peptide mutagenesis defined the biochemical rules — long but less hydrophobic H-regions and downstream positive charges — that create SEC63 dependence, explaining substrate selectivity.

   Evidence Proteomics in intact human cells, in vitro translocation, signal peptide mutagenesis

   PMID:32133789

   Open questions at the time

   • Whether these rules apply universally across cell types and species was not tested
   • Structural basis for how suboptimal signal peptides engage SEC63-activated SEC61 was unknown
10. 2021 High

    Cryo-EM structures of the SEC61–SEC62–SEC63 complex revealed that SEC63 partially opens the SEC61 lateral gate while SEC62 displaces the plug, providing an atomic-level mechanism for how the auxiliary translocon activates the channel.

    Evidence Cryo-EM at near-atomic resolution, molecular dynamics simulations, mutagenesis of contact residues in yeast

    PMID:33398175

    Open questions at the time

    • Structures of the complex engaged with a translocating substrate are lacking
    • How BiP/J-domain action is coordinated with gate opening at the structural level remains unresolved
11. 2023 Medium

    Discovery that IRE1α phosphorylates SEC63 at T537 under ER stress, promoting ACLY stabilization and nuclear translocation to drive lipid biosynthesis and epigenetic activation of Snail1, suggested a non-canonical role for SEC63 in cancer metastasis.

    Evidence GST pull-down, IP-MS, phosphorylation/ubiquitination assays, RNA-seq, metabolite detection in HCC cell lines

    PMID:37122003

    Open questions at the time

    • Nuclear translocation of an ER transmembrane protein lacks mechanistic explanation for membrane release
    • Whether the SEC63–ACLY axis operates outside hepatocellular carcinoma contexts is unknown
    • Independent replication is needed

• Key unresolved questions include the structural basis of SEC63 J-domain–BiP coupling during active translocation, the direct versus indirect nature of the SEC63–polycystin-1 interaction, and whether SEC63 phosphorylation and palmitoylation integrate to tune translocon activity under physiological stress.
• No structure of the active SEC63–BiP–substrate ternary complex exists
• Palmitoylation of SEC63 at C490 has not been validated by orthogonal acyl-capture assays
• Two-hit versus haploinsufficiency mechanism in PCLD remains genetically unresolved