UFC1

UFC1 is the E2-like conjugating enzyme of the ufmylation pathway, accepting activated UFM1 from the E1 enzyme UBA5 and relaying it to substrate proteins (PMID:29868776, PMID:34588452). It comprises a canonical E2 catalytic core plus a stabilizing N-terminal domain, and carries out thioester chemistry through its active-site Cys116, which sits in a flexible, solvent-exposed loop positioned to capture the C-terminal Gly83 of UFM1 (PMID:17825256, PMID:19101823). Charging of UFC1 depends on an unusual mode of E1 docking: a short linear sequence in the unstructured C-terminus of UBA5 (its last ~20 residues, within region 364–404) binds UFC1 and accelerates transthiolation, with part of UBA5 reaching toward the UFC1 active-site Cys to compensate for a loop absent in UFC1 but present in other E2s (PMID:34588452, PMID:34299007, PMID:25084390). Hand-off to substrates is directed by the E3 ligase UFL1, whose N-terminal helix engages a conserved interface on UFC1 and competes with UBA5 for binding; this competition shifts toward UFL1 once UFC1 is UFM1-charged, coupling E1 release to E3 engagement (PMID:37988244). Catalysis is mediated by an atypical TAK motif (in place of the canonical HPN motif) that builds two distinct oxyanion holes—one supporting E3-driven trans-ufmylation and one for cis auto-ufmylation (PMID:40280917). Biallelic loss-of-function UFC1 mutations that impair UFM1–UFC1 thioester formation cause severe early-onset encephalopathy with progressive microcephaly by globally reducing cellular ufmylation (PMID:29868776).

1. 2007 High

   Establishing the architecture of UFC1 was the first step in defining how this enzyme could function within the ufmylation cascade, by identifying both its E2 core and where it engages the upstream E1.

   Evidence X-ray crystallography at 1.6 Å with site-directed mutagenesis mapping the UBA5 binding site by comparison to Ubc12

   PMID:17825256

   Open questions at the time

   • Did not define the catalytic active-site residue or thioester chemistry
   • UBA5 binding mode inferred by comparison, not from a complex structure
2. 2008 High

   Identifying Cys116 in a flexible loop poised to receive UFM1 established the catalytic basis for UFC1's E2 thioester chemistry.

   Evidence NMR and X-ray structures plus modeling of the UFC1–UFM1 complex positioning UFM1 Gly83 near Cys116

   PMID:19101823

   Open questions at the time

   • The UFC1–UFM1 complex was modeled, not experimentally solved
   • Role of the N-terminal helix conformational change in specificity not functionally tested
3. 2014 Medium

   Mapping the minimal UBA5 segment needed for UFC1 recognition versus transthiolation separated the binding determinant from the transfer-competent region, beginning to explain the unusual E1–E2 interaction.

   Evidence Deletion mutagenesis with biochemical transthiolation assays and preliminary crystallization of a UBA5 C-terminus–UFC1 fusion

   PMID:25084390

   Open questions at the time

   • Full crystal structure of the complex not reported (preliminary 3.0 Å)
   • Atomic basis of why a region outside the binding determinant is needed for transfer unresolved
4. 2014 Low

   A candidate substrate link was explored by showing UFC1 interacts with and co-localizes with the NCAM140 cytoplasmic domain, raising the possibility of ufmylation of a cell-surface protein.

   Evidence Protein macroarray, co-localization imaging, and NCAM140 endocytosis assays

   PMID:24726913

   Open questions at the time

   • Ufmylation of NCAM140 was not directly demonstrated
   • Single lab, macroarray-based interaction without reciprocal validation
   • Functional relevance to ufmylation pathway unclear
5. 2018 High

   Patient mutations that block UFM1–UFC1 thioester formation tied UFC1 enzymatic activity directly to human brain development, establishing it as the essential E2 of ufmylation in vivo.

   Evidence Biochemical thioester intermediate assays with patient-derived mutations across multiple families

   PMID:29868776

   Open questions at the time

   • Substrates whose loss of ufmylation drives the neurological phenotype not identified
   • Tissue-specific reasons for brain vulnerability unresolved
6. 2021 High

   Solving the UFC1–UBA5 C-terminus complex revealed a short linear docking motif unique among E1–E2 pairs and showed how UBA5 substitutes for a missing UFC1 loop to enable transfer.

   Evidence X-ray crystallography of a UFC1–UBA5 C-terminal peptide complex plus NMR, ITC, and biochemical transfer assays (two independent studies)

   PMID:34299007 PMID:34588452

   Open questions at the time

   • Dynamics of the charged intermediate during transfer not visualized
   • How UBA5 release is triggered after charging not defined here
7. 2023 High

   Defining the UFL1 N-terminal helix interface on UFC1 and its competition with UBA5 explained how UFM1-charging switches the enzyme from E1 acceptor to E3-directed donor.

   Evidence Crystal structure of a UFL1–DDRGK1 fusion with UFC1, plus NMR and biochemical competition assays

   PMID:37988244

   Open questions at the time

   • Structure of the full E2–E3–substrate transfer complex not resolved
   • How charging mechanistically increases UFL1 affinity not detailed at atomic level
8. 2025 High

   Discovery of UFC1's atypical TAK motif and dual oxyanion holes explained how a single E2 supports both E3-driven trans-ufmylation and cis auto-ufmylation.

   Evidence Crystal structures of UFC1 mutants including an oxyanion-intermediate mimic, with in vitro ufmylation assays

   PMID:40280917

   Open questions at the time

   • Physiological balance between trans- and cis-ufmylation not quantified in cells
   • Substrate determinants selecting each pathway unknown
9. 2025 Medium

   Linking UFC1 to orthoflavivirus assembly placed ufmylation at a post-replication step required for infectious virion production.

   Evidence siRNA knockdown of UFC1 (and other ufmylation components) with viral infectivity and RNA replication assays (preprint)

   PMID:bio_10.1101_2025.01.09.632082

   Open questions at the time

   • Preprint, single lab
   • The ufmylated substrate mediating the assembly defect not identified
   • Direct evidence of ufmylation at the assembly step lacking
10. 2025 Medium

    A disease-associated missense mutation was shown to mislocalize UFC1 and disrupt oligodendroglial differentiation signaling, connecting UFC1 dysfunction to myelination defects.

    Evidence Mutant UFC1 expression in a differentiating oligodendroglial cell line with immunofluorescence and Western blotting for Akt phosphorylation and myelination markers

    PMID:39846712

    Open questions at the time

    • Single cell model, single lab
    • Whether mislocalization reflects loss of ufmylation activity not established
    • Link between Akt signaling and ufmylation not mechanistically defined

• The physiological substrates ufmylated by UFC1 and how their modification underlies neurodevelopmental phenotypes and viral assembly remain unresolved.
• No validated endogenous substrate identified
• Mechanism linking global ufmylation loss to tissue-specific defects unknown
• Cellular localization context of ufmylation activity uncharacterized