ANKIB1

Model summary

A soluble human E3 ubiquitin ligase that assembles substrates through ankyrin-repeat scaffolding and catalyzes ubiquitin transfer via a RING–IBR–Rcat core. It builds K48-linked polyubiquitin chains that are preferentially recognized by the proteasome, thereby driving ubiquitin-dependent protein turnover. By coordinating cytosolic and nucleoplasmic complexes and engaging ubiquitin-binding motifs that bias chain selection and proteasome delivery, it promotes proteostasis and can tip cell-fate decisions toward apoptosis under stress or checkpoint activation.

Predicted Gene Ontology terms

1. 01

   I begin by dissecting the InterPro architecture and its order along the polypeptide.

   The N-terminus is dominated by ankyrin-repeat scaffolding: IPR036770 (Ankyrin repeat-containing domain superfamily, residues 3–203) encompasses tandem repeats annotated explicitly as IPR002110 Ankyrin repeat [45–77] and [136–194]. Ankyrin repeats create elongated, solenoid-like surfaces that enforce selective protein binding and assembly of regulatory complexes, directly supporting GO:0005515 protein binding as a core molecular function. This N-terminal scaffold transitions into a canonical RBR/TRIAD E3 ligase core. A TRIAD supradomain, IPR044066 [329–569], spans the central region and houses a RING module: IPR013083 (Zinc finger, RING/FYVE/PHD-type superfamily, [328–415]) and IPR001841 (Zinc finger, RING-type, [333–381]) together with the lineage-specific IPR047563 (RING finger, HC subclass, [331–388]). Immediately downstream lies the IBR architecture: IPR002867 IBR domain [402–478] and a second IBR annotation [501–565], which are hallmarks of RBR E3 ligases that coordinate catalysis between a RING and an Rcat. The C-terminal catalytic half is reinforced by IPR047564 (Rcat domain, [508–569]) and an extended RING annotation IPR001841 [519–643], consistent with the bipartite RING/Rcat chemistry. The entire catalytic block is classified by IPR031127 (E3 ubiquitin ligase RBR family, [242–609]), confirming an RBR-type E3 mechanism. A downstream IPR045840 Ariadne domain [597–713] often stabilizes RBR assemblies and can mediate RNA/protein-associated interactions that tune ubiquitination outcomes. Finally, IPR003903 (Ubiquitin interacting motif, [851–870]) provides a short α-helical UIM that binds mono- and polyubiquitin, biasing chain selection and coupling the ligase to downstream fates.

2. 02

   This ordered architecture dictates molecular function.

   The ankyrin array recruits substrates and adaptors; the RING–IBR–Rcat supradomain executes E3 ubiquitin transfer via a transient thioester on the Rcat; and the UIM binds ubiquitin chains to steer linkage type. Such an arrangement causes robust protein binding (GO:0005515) and confers ubiquitin ligase activity that builds specific polyubiquitin topologies. The presence of a TRIAD/RBR core and UIM strongly supports assembly of K48-linked chains, aligning with GO:0070936 protein K48-linked ubiquitination and with proteasome-directed turnover.

3. 03

   From molecular function, the biological processes follow.

   K48-linked polyubiquitination targets substrates to the 26S proteasome, directly situating the protein in GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process. By promoting proteasomal clearance of signaling and cell-cycle regulators, the ligase can tip cell-fate decisions toward apoptosis under stress or checkpoint activation, consistent with GO:0043065 positive regulation of apoptotic process. The ankyrin scaffold and nucleoplasmic access enable assembly of nuclear complexes that modulate transcriptional and checkpoint pathways, while cytosolic pools handle cytoplasmic substrates.

4. 04

   Cellular location is inferred from the soluble, multidomain scaffold and absence of transmembrane segments.

   The ankyrin and RBR cores are soluble, and the UIM-mediated ubiquitin engagement is compatible with both cytosolic and nuclear ubiquitin circuits. This supports localization to GO:0005829 cytosol and GO:0005654 nucleoplasm, matching the need to regulate both cytoplasmic and nuclear substrates.

5. 05

   Mechanistically, the protein likely operates as follows: ankyrin repeats nucleate substrate/adaptor assemblies; the RING–IBR–Rcat core catalyzes ubiquitin transfer to build K48-linked chains; the UIM binds and biases chain topology and handoff to the proteasome; and the Ariadne domain stabilizes higher-order assemblies that tune activity.

   I hypothesize transient interactions with ubiquitin-conjugation machinery and proteasome receptors: E2 conjugases that feed RBR ligases; ubiquitin itself via the UIM; and proteasome ubiquitin receptors that accept K48-tagged cargo. In the nucleus and cytosol, this ensemble would preferentially route specific regulatory proteins toward degradation, thereby coupling ubiquitin signaling to proteostasis and apoptosis pathways.