ERG28

ERG28 (C14orf1/NET51) encodes a conserved multi-pass endoplasmic reticulum transmembrane protein that acts as a non-catalytic scaffold organizing sterol biosynthetic enzymes (PMID:12119386, PMID:36216146). In S. cerevisiae, Erg28p is required for sterol C-4 demethylation, and its deletion causes accumulation of 3-keto and carboxylic acid sterol intermediates that phenocopy erg26 and erg27 mutants (PMID:11160377). It physically tethers the C-4 demethylation enzymes Erg25p/Erg26p/Erg27p in the ER and bridges them to the downstream C-24 methyltransferase Erg6p, whose ER abundance drops when ERG28 is lost (PMID:12119386, PMID:15522820); a systematic membrane two-hybrid screen places Erg28p at the center of a broader sterol-enzyme network including Erg11p and Erg1p (PMID:15995173), and a cytoplasmic-loop motif spanning residues 63–72 within a four-helix bundle is required for complex assembly (PMID:29319811). The human ortholog ERG28 is transcriptionally driven by SREBP-2, self-associates, and interacts with the cholesterol synthesis enzymes NSDHL and SC4MOL; its loss reduces cholesterol synthesis and impairs SREBP-2 activation, establishing a conserved role in cholesterol biosynthesis (PMID:36216146). Beyond sterol metabolism, ERG28 orthologs perform additional ER-membrane scaffolding functions: in C. elegans, ERG-28 promotes trafficking of SLO-1 BK channels to synaptic membranes by shielding them from DDI-1-dependent proteasomal degradation (PMID:28168949), and in fission yeast Erg28 binds the Mto1-Mto2/γ-TuSC microtubule-nucleation machinery and restrains microtubule assembly from the ER surface via its cytosolic N-terminus (PMID:41989917).

1. 2001 Medium

   Establishing whether ERG28 participates in sterol biosynthesis, the discovery that its deletion blocks C-4 demethylation defined it as a functional component of the ergosterol pathway rather than an incidental ER protein.

   Evidence GC-MS sterol profiling of ERG28-deleted yeast compared to erg26/erg27 nulls

   PMID:11160377

   Open questions at the time

   • Mechanism inferred from phenotypic similarity, not direct interaction in this study
   • Did not establish whether Erg28p is catalytic or scaffolding
2. 2002 High

   To resolve how Erg28p acts without enzymatic activity, physical interaction and localization data showed it is an ER membrane protein that binds Erg27p and Erg25p, defining it as a transmembrane scaffold tethering the demethylation complex.

   Evidence Differential centrifugation, GFP localization, reciprocal Co-IP, sucrose gradient and affinity purification in S. cerevisiae

   PMID:12119386

   Open questions at the time

   • Did not map the interacting residues on Erg28p
   • Stoichiometry of the complex not resolved
3. 2004 High

   Extending the scaffold model downstream, Erg28p was shown to bridge the C-4 demethylation complex to Erg6p, explaining how sequential pathway enzymes are coupled in the ER.

   Evidence ER fraction Western blots, reciprocal Co-IP, and split-ubiquitin membrane two-hybrid in erg28 vs. wild-type yeast

   PMID:15522820

   Open questions at the time

   • Whether Erg6p destabilization is direct or secondary to complex disruption not distinguished
4. 2005 High

   A systematic pairwise interaction screen tested how broad the scaffold's reach is, revealing Erg28p contacts six sterol enzymes and is most tightly associated with Erg27p/Erg25p/Erg11p/Erg6p, casting it as the organizer of a large sterol biosynthetic enzyme assembly.

   Evidence Split-ubiquitin membrane two-hybrid across all 14 sterol enzymes with Co-IP confirmation

   PMID:15995173

   Open questions at the time

   • Did not establish whether interactions are simultaneous within one complex or mutually exclusive
   • No structural architecture of the assembly
5. 2013 Medium

   Addressing how ERG28 expression is controlled in yeast, a cis-regulatory promoter mutation was shown to abolish Sok2/Mot3 binding and down-regulate the ergosterol pathway, linking ERG28 dosage to azole drug resistance.

   Evidence Promoter deletion mapping, transcription factor binding, and fluconazole phenotyping in engineered yeast

   PMID:24068973

   Open questions at the time

   • Did not test ERG28 protein-level consequences directly
   • Single-lab regulatory mapping
6. 2018 Medium

   To pinpoint the structural basis of scaffolding, mutagenesis identified a 63–72 cytoplasmic-loop motif within a four-helix bundle as essential for complex assembly and demethylation.

   Evidence Serial truncation mutagenesis, GC-MS sterol profiling, complementation, and homology modeling in yeast

   PMID:29319811

   Open questions at the time

   • No experimental structure to confirm the modeled four-helix bundle
   • Which partner each motif residue contacts not defined
7. 2022 High

   Determining whether the yeast scaffold role is conserved in humans, ERG28 was shown to be SREBP-2-driven, to bind NSDHL and SC4MOL, and to be required for efficient cholesterol synthesis and SREBP-2 activation.

   Evidence qRT-PCR, luciferase/ChIP-seq promoter analysis, split-luciferase interaction, CRISPR KO with rescue, and radiolabeled flux in Huh7 cells

   PMID:36216146

   Open questions at the time

   • Whether human ERG28 tethers the full enzyme assembly as in yeast not directly shown
   • Mechanism of impaired SREBP-2 activation unresolved
8. 2017 High

   Revealing a non-sterol scaffolding function, the C. elegans ortholog ERG-28 was shown to protect SLO-1 BK channels from DDI-1-dependent degradation, enabling their ER-to-synapse trafficking.

   Evidence Genetic epistasis (erg-28;ddi-1 double mutants), SLO-1 localization microscopy, and behavioral/neurotransmitter assays in C. elegans

   PMID:28168949

   Open questions at the time

   • Whether ERG-28 directly binds SLO-1 or acts indirectly not established
   • Relationship to its sterol role in worms not addressed
9. 2026 High

   Uncovering a further ER-surface function, fission yeast Erg28 was shown to bind Mto1-Mto2/γ-TuSC and inhibit microtubule nucleation, linking ER membrane scaffolding to cytoskeletal control.

   Evidence Biochemical binding assays, in vitro microtubule assembly reconstitution, domain mapping, and deletion-mutant imaging in S. pombe

   PMID:41989917

   Open questions at the time

   • Whether this microtubule role is conserved in mammals unknown
   • Mechanistic link between sterol scaffolding and microtubule restraint unresolved

• Whether the divergent ERG28 functions — sterol enzyme scaffolding, BK channel protection, and microtubule restraint — reflect a single unifying biochemical activity or independent moonlighting roles remains unresolved.
• No experimental structure of any ERG28 complex
• Human ortholog's potential channel-trafficking or microtubule roles untested
• Failure of human ERG28 to complement yeast erg28 (#11) suggests functional divergence not yet mechanistically explained