NCLN

Nicalin (NCLN) is a core ER-resident transmembrane protein that organizes a hierarchical sub-complex with NOMO and TMEM147 embedded within a ribosome-associated multipass translocon that supports the biogenesis of multi-pass membrane proteins (PMID:20538592, PMID:32820719). Nicalin acts as the limiting, stabilizing factor for the sub-complex: it first forms a mutually stabilizing intermediate with NOMO, after which TMEM147 joins, with each partner becoming unstable in the absence of Nicalin (PMID:17261586, PMID:20538592). Within the native ER membrane, this Nicalin-TMEM147-NOMO module assembles into a ~360 kDa multipass translocon together with Sec61, TMCO1, and CCDC47, organized around a central membrane cavity at the ribosome exit tunnel; NOMO bridges the luminal domains of Nicalin and TRAPalpha, and loss of accessory components reduces levels of the multi-pass client EAAT1 (PMID:32820719, PMID:38866426). The translocon-associated chaperone SigmaR1 directly binds Nicalin, an interaction strengthened by phosphatidylcholine binding (PMID:41476177). Independent of its translocon role, the Nicalin-NOMO complex antagonizes Nodal/Activin (TGFbeta) signaling during vertebrate mesendodermal patterning, and the Nicalin ortholog is required for enteric nervous system development in zebrafish (PMID:15257293, PMID:28274275). Nicalin also participates in ER quality control of membrane clients, governing the ER-versus-surface distribution of DEG/ENaC channels through its C. elegans ortholog NRA-2 (PMID:24567339).

1. 2004 High

   Established Nicalin's first biological function by showing it partners with NOMO to antagonize Nodal/TGFbeta signaling during embryonic patterning.

   Evidence Zebrafish ectopic expression and morpholino knockdown, cell-based Nodal/Activin reporter assay, and co-immunoprecipitation

   PMID:15257293

   Open questions at the time

   • Molecular mechanism by which the Nicalin/NOMO complex inhibits Nodal signaling not defined
   • Relationship between this signaling role and ER residence unresolved
2. 2007 Medium

   Defined Nicalin as the assembly-limiting and stabilizing subunit of the Nicalin-NOMO complex, explaining how complex stoichiometry is controlled post-transcriptionally.

   Evidence RNAi knockdown, overexpression, and protein stability assays with mRNA controls in cultured cells

   PMID:17261586

   Open questions at the time

   • Degradation pathway for excess NOMO not identified
   • Structural basis of mutual stabilization unknown
3. 2010 High

   Extended the complex to a third subunit by identifying TMEM147 and showing hierarchical assembly (Nicalin-NOMO first, then TMEM147), placing all three in the ER.

   Evidence Affinity purification of native complex with mass spectrometry, overexpression/RNAi, immunofluorescence, and zebrafish expression analysis

   PMID:20538592

   Open questions at the time

   • Functional output of the three-subunit complex not yet defined at this stage
   • Interfaces between TMEM147 and the Nicalin-NOMO intermediate not mapped
4. 2014 Medium

   Linked Nicalin to ER quality control of membrane clients by showing its ortholog NRA-2 controls ER-versus-surface distribution of a DEG/ENaC channel in a subunit-composition-dependent manner.

   Evidence C. elegans RNAi and knockout genetics, neuronal localization assays, and two-electrode voltage clamp in Xenopus oocytes

   PMID:24567339

   Open questions at the time

   • Mechanism of selective retention of mutant channels unknown
   • Generality to human NCLN clients not established
5. 2014 Low

   Reported Nicalin as a ganglioside GT1b-binding protein associated with an AMPA receptor trafficking complex, raising a possible neuronal trafficking role.

   Evidence Affinity capture with GT1b, quantitative proteomic mass spectrometry, and ATPgammaS competition in cerebellar granule neurons

   PMID:25253868

   Open questions at the time

   • Single affinity-capture/MS experiment with no functional follow-up specific to Nicalin
   • Direct versus indirect association with AMPAR complex not resolved
6. 2017 Medium

   Demonstrated an in vivo developmental requirement for NCLN in enteric nervous system formation.

   Evidence Morpholino knockdown and CRISPR knockout in zebrafish with ENS phenotype analysis

   PMID:28274275

   Open questions at the time

   • Cellular pathway connecting NCLN loss to ENS defects unknown
   • Whether the defect reflects translocon or Nodal-modulating function unclear
7. 2020 High

   Resolved the central function by placing Nicalin within a ~360 kDa ribosome-associated multipass translocon and tying its sub-complex to biogenesis of multi-pass membrane proteins.

   Evidence Cryo-EM structure, ribosome profiling, and genetic knockout of accessory components with EAAT1 protein-level readout

   PMID:32820719

   Open questions at the time

   • Specific contribution of Nicalin to client folding versus retention not isolated
   • Range of physiological multi-pass clients incompletely defined
8. 2020 Low

   Identified a candidate ER quality-control client, sEphA7, whose secretion Nicalin restricts.

   Evidence IP/MS identification, immunofluorescence co-localization in HEK293 cells, and subcellular fractionation

   PMID:32914261

   Open questions at the time

   • No mutagenesis or rescue to confirm direct effect
   • Mechanism of restricted ER entry and secretion not established
9. 2024 Medium

   Defined the architecture of the sub-complex in native membranes, showing NOMO bridges the luminal domains of Nicalin and TRAPalpha and that translocon composition tracks ribosome activity.

   Evidence Cryo-electron tomography, subtomogram averaging, and AlphaFold modeling in native ER membranes

   PMID:38866426

   Open questions at the time

   • Contacts not validated by mutagenesis
   • Functional consequence of activity-dependent composition changes unknown
10. 2025 Medium

    Added SigmaR1 as a direct, lipid-regulated Nicalin partner within the translocon.

    Evidence Endogenous tagging, cell fractionation, co-immunoprecipitation, and lipid-binding assay

    PMID:41476177

    Open questions at the time

    • Functional consequence of SigmaR1-Nicalin association for client biogenesis not shown
    • Single-lab biochemistry without structural mapping of the interface

• How Nicalin's distinct activities — translocon-assisted multi-pass protein biogenesis, ER quality control of specific clients, and Nodal antagonism — are mechanistically related or separable remains unresolved.
• No defined molecular activity assigned to Nicalin's luminal domain
• Client specificity rules unknown
• Connection between developmental signaling roles and translocon function unestablished