HEATR4

Model summary

A cytoplasmic and cytoskeletal scaffold that uses an extended armadillo/HEAT-repeat solenoid to mediate multivalent protein binding and assemble mitotic machinery. By concentrating motors and spindle assembly factors, it promotes robust spindle formation, aligns chromosomes at the metaphase plate, and directs components toward centrosomes. A regulated nuclear pool likely reflects assembly-state cycling during mitosis. Overall, it functions as a versatile organizer that stabilizes the mitotic apparatus and coordinates chromosome alignment.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The C-terminal half of the protein is dominated by tandem armadillo/HEAT-like solenoids: IPR016024 (Armadillo-type fold, residues 469–883) encompasses two nested helical superstructures, IPR011989 (Armadillo-like helical, residues 476–697) followed by IPR011989 (Armadillo-like helical, residues 698–885). Within this solenoidal core sits a discrete IPR000357 HEAT repeat (residues 530–558). Armadillo and HEAT repeats form elongated, right-handed superhelices that create extensive protein–protein interaction surfaces rather than catalytic centers. This architecture causally enforces multivalent binding and scaffolding, which directly supports GO:0005515 protein binding as the molecular function.

2. 02

   A solenoidal scaffold of this type typically organizes large cytoskeletal assemblies.

   The presence of a HEAT repeat embedded in an extended armadillo-type fold provides a flexible platform to capture and position microtubule-associated assemblies that drive mitosis. Such scaffolds commonly coordinate motor proteins and checkpoint factors to align chromosomes and build a bipolar apparatus. This structural logic explains several mitotic processes: it stabilizes and positions the bipolar apparatus underlying GO:0007052 mitotic spindle organization; it concentrates force-producing machinery that aligns chromosomes at the equator, accounting for GO:0007080 mitotic metaphase plate congression; and it provides docking sites that bias components toward centrosomes, consistent with GO:0071539 protein localization to centrosome. The same solenoid scaffold can also mediate nucleo-cytoplasmic shuttling and assembly-state sensing, which rationalizes a nuclear pool and supports the provided cellular component label GO:0005634 nucleus.

3. 03

   The subcellular distribution follows from this dual role.

   The armadillo/HEAT scaffold assembles and stabilizes spindle-associated complexes in the cytoplasm and cytoskeleton, matching the observed cytoplasmic and cytoskeletal residence. A nuclear fraction is expected for assembly factors that cycle on and off chromatin and the nuclear envelope during mitosis, aligning with the nucleus label. Together, these features place the protein at cytoplasm–cytoskeleton interfaces with a regulated nuclear subpopulation.

4. 04

   Mechanistically, the solenoidal repeats create a high-capacity binding surface that recruits and positions microtubule regulators and motors to ensure robust spindle assembly and chromosome alignment.

   I hypothesize that the scaffold captures dynein–dynactin and kinesin assemblies to generate poleward forces and maintains centrosomal enrichment via interactions with pericentriolar matrix components. Likely partners include microtubule organizers and motors (e.g., dynein–dynactin, NuMA, and kinesin-13), spindle assembly factors (e.g., TPX2 pathway components), and centrosomal organizers (e.g., CEP proteins). This cooperative network explains how the scaffold simultaneously drives spindle organization, congression, and centrosome-directed protein localization.