CAND1 (Q86VP6) research notes

Identity / domain architecture

Cullin-associated NEDD8-dissociated protein 1 (CAND1), formerly TIP120A (TBP-interacting protein 120A). A ~120 kDa HEAT-repeat protein that clamps around the cullin scaffold. PMID:15537541

UniProt summary: key assembly factor of SCF E3 ligases that promotes exchange of the substrate-recognition F-box subunit; acts as an F-box protein exchange factor; activity coupled to neddylation cycles; in the deneddylated state cullin-binding CAND1 binds CUL1-RBX1, increasing SCF dissociation and promoting F-box exchange; probably similar role in other CRLs. [file:human/CAND1/CAND1-uniprot.txt "Key assembly factor of SCF (SKP1-CUL1-F-box protein) E3\nCC ubiquitin ligase complexes that promotes the exchange of the substrate-\nCC recognition F-box subunit"] [file:human/CAND1/CAND1-uniprot.txt "Acts as a F-box protein\nCC exchange factor."]
CAND1 binds unneddylated CUL1 and regulates SCF formation; dissociation of CAND1 promotes F-box incorporation. PMID:12504026
Neddylation of CUL1 (or SKP1+ATP) dissociates CAND1. PMID:12504026
CAND1 selectively binds unneddylated CUL1 and is dissociated by neddylation; forms CUL1-ROC1 ternary complex; regulates assembly of productive SCF ligases. PMID:12504025
Associates with all cullins tested; forms a complex with CUL1 and Rbx1 but interferes with Skp1/F-box binding; reduces SCF ubiquitination of substrate; "negative regulator of SCF E3 ubiquitin ligases." PMID:12609982

Crystal structure of Cand1-Cul1-Roc1: Cand1 superhelix clamps Cul1; a beta-hairpin occupies the adaptor (Skp1) binding site; HEAT repeats bury the Cul1 lysine whose neddylation blocks Cand1 association. PMID:15537541

Although early biochemistry framed CAND1 as an inhibitor/sequester, in vivo it is a positive regulator of CRL activity that promotes substrate-receptor exchange (dynamic recycling). PMID:21249194
In mammalian cells CAND1 is predominantly cytoplasmic and cullins are its major interactors; only small amounts bind Cul1 at steady state (consistent with transient exchange-factor action). PMID:21249194
F-box protein + substrate promote dissociation of the cullin-CAND1 complex, coupling assembly to substrate availability and neddylation. PMID:16861300

Predominantly cytoplasmic/cytosolic, with nuclear pools (CRL4 in nucleus per Reactome). Cytosol and nucleus are both supported. Exosome/membrane/secretory-granule/ficolin-granule annotations are high-throughput proteomic (HDA) or Reactome "exocytosis" assignments and are best treated as over-annotations / contaminant-type localizations rather than functional sites.

CAND1 was originally described as TIP120A, a TBP-interacting transcriptional regulator. PMID:10581176
The TBP-class protein binding (GO:0017025), positive regulation of DNA-templated transcription (GO:0045893), positive regulation of RNA pol II PIC assembly (GO:0045899), and cell differentiation (GO:0030154, from PMID:10581176) annotations derive from this early work. They predate the CRL-assembly paradigm and are not the consensus core function; treat as non-core / over-annotated.

CORE: F-box/substrate-receptor exchange factor & SCF/CRL assembly regulation (GO:0010265 SCF complex assembly; involvement in protein ubiquitination GO:0016567 as a regulator, not catalyst); binding to / being part of cullin-RING ligase complex (GO:0031461) in its unneddylated/exchange state; negative regulation of (premature/unproductive) CRL catalytic activity (GO:0043086); cytosol & nucleus localization.
NON-CORE / historical: TBP-class binding and transcription-regulation terms, cell differentiation (TIP120A era).
OVER-ANNOTATED / uninformative: many high-throughput "protein binding" (GO:0005515) IPI hits; extracellular region, secretory granule lumen, ficolin-1-rich granule lumen, extracellular exosome, membrane (HDA/Reactome contaminant-type localizations).
NOTE: CAND1 is a regulator/assembly factor, NOT a catalytic E3 or a ubiquitin-transfer enzyme; protein ubiquitination annotations are "involved_in" (regulatory) and should be read as such.