SCOC

SCOC is a Golgi-localized short coiled-coil protein that regulates autophagosome formation during amino acid starvation, identified through a genome-wide siRNA screen as required for starvation-induced autophagy (PMID:22354037). Mechanistically, SCOC forms a starvation-sensitive trimeric complex with the ULK1-binding protein FEZ1 and UVRAG, positioning it to modulate ULK1 and Beclin 1 complex activities (PMID:22354037). Structurally, SCOC assembles as a parallel left-handed coiled-coil homodimer whose conformational flexibility and surface residue R117 underpin its stable interaction with the coiled-coil domain of FEZ1, which itself homodimerizes in an anti-parallel topology to yield a heterotetrameric FEZ1/SCOC assembly (PMID:24098481, PMID:24116125). SCOC additionally carries a LIR motif that binds ATG8-family proteins with preference for GABARAP, GABARAPL1, LC3A, and LC3C, and phosphorylation within and around this motif by the autophagy kinases ULK1-3 and TBK1 tunes its affinity and specificity toward LC3 family members (PMID:33845085). The function is conserved: the C. elegans ortholog UNC-69 cooperates with the FEZ1 ortholog UNC-76 to control axon outgrowth, guidance, fasciculation, and presynaptic vesicle organization (PMID:16725058).

1. 2006 High

   Established the first physiological role and a conserved binding partner for the SCOC ortholog, showing it is not an orphan coiled-coil protein but a neuronal regulator acting with FEZ1.

   Evidence C. elegans loss-of-function genetics, co-immunoprecipitation, colocalization imaging, and vertebrate RNAi knockdown

   PMID:16725058

   Open questions at the time

   • Molecular mechanism by which the UNC-69/UNC-76 complex directs axon outgrowth not defined
   • Relationship to autophagy not yet recognized
   • Human SCOC function not directly tested
2. 2012 High

   Connected SCOC to autophagy, answering what cellular pathway it operates in by placing it in a starvation-sensitive trimeric complex with FEZ1 and UVRAG required for autophagosome formation.

   Evidence Genome-wide siRNA screen with GFP-LC3 readout and reciprocal co-immunoprecipitation, Golgi localization

   PMID:22354037

   Open questions at the time

   • Direct enzymatic effect on ULK1 or Beclin 1 complexes not demonstrated
   • How starvation sensitizes complex assembly unresolved
   • Stoichiometry of the trimeric complex not defined
3. 2013 High

   Resolved the molecular architecture of SCOC and its FEZ1 interaction, showing how a flexible parallel coiled-coil homodimer engages an anti-parallel FEZ1 dimer to form a heterotetramer.

   Evidence X-ray crystallography, MALLS, native MS, NMR, cross-linking MS, SAXS, and core/surface residue mutagenesis across two studies

   PMID:24098481 PMID:24116125

   Open questions at the time

   • Structure of the full SCOC/FEZ1/UVRAG trimeric complex not determined
   • Functional consequence of conformational flexibility in cells not tested
   • How UVRAG joins the assembly unknown
4. 2021 High

   Identified SCOC as a direct ATG8 effector through a LIR motif and showed that autophagy-kinase phosphorylation dynamically controls its ATG8 selectivity, providing a regulatory switch linking it to membrane-associated ATG8 proteins.

   Evidence Structural and biophysical binding assays, in vitro kinase assays with ULK1-3 and TBK1, and LIR mutagenesis

   PMID:33845085

   Open questions at the time

   • In-cell relevance of phospho-regulated ATG8 binding to autophagosome formation not established
   • Which kinase acts on SCOC under physiological starvation unclear
   • Link between LIR-mediated ATG8 binding and the FEZ1/UVRAG complex unresolved
5. 2022 Medium

   Extended the SCOC interaction network to a larger FEZ1/SCOC/ULK1/NBR1 complex and placed it under microRNA control in neuronal cells.

   Evidence Co-immunoprecipitation and dual-luciferase reporter assay for miR-129-5p targeting

   PMID:35435132

   Open questions at the time

   • Single Co-IP without reciprocal validation for the four-protein complex
   • Direct versus indirect SCOC contacts within the complex not dissected
   • Functional autophagy consequence attributable specifically to SCOC not isolated

• Whether SCOC's LIR-mediated ATG8 binding, its FEZ1/UVRAG complex, and its conserved neuronal role are mechanistically integrated into a single regulatory step in autophagosome biogenesis remains unresolved.
• No structure of the assembled multiprotein complex with ATG8
• Causal in-cell pathway from SCOC phosphorylation to autophagosome formation not traced
• Direct biochemical effect on ULK1/Beclin 1 activity untested