LRCH3

Model summary

A cytoplasmic adaptor that combines an N-terminal leucine-rich repeat scaffold with a C-terminal calponin-homology module to assemble and position cytoskeletal complexes. The LRR array recruits partner proteins, while the calponin-homology fold anchors assemblies to actin-rich regions, thereby guiding the organization of septin-based structures. Operating as a soluble scaffold in the cytosol, it likely coordinates the assembly and maintenance of filament systems by concentrating septins and associated factors at appropriate cortical sites.

Predicted Gene Ontology terms

1. 01

   I begin with the domain architecture.

   The N-terminal half is dominated by a leucine-rich repeat (LRR) scaffold: IPR032675 (Leucine-rich repeat domain superfamily, residues 46–285) and IPR050216 (Leucine-rich repeat domain-containing protein family, residues 49–494) define a curved solenoid platform optimized for avid, selective protein–protein interactions. This platform is reinforced by multiple tandem LRR units—IPR001611 (Leucine-rich repeat, residues 88–140), IPR003591 typical LRR repeats at 104–126, 127–150, 172–194, 195–218, and 240–263—that collectively create a continuous binding surface. Such an N-terminal LRR array causes high-capacity scaffolding and substrate capture, a hallmark of cytoskeletal adaptors.

2. 02

   The C-terminal half switches to an actin-binding module: IPR036872 (CH domain superfamily, residues 649–776) and its core IPR001715 (Calponin homology domain, residues 652–765) specify a calponin-homology fold that binds actin and actin-associated assemblies.

   The ordered architecture—LRR scaffold followed by a CH domain—creates a bipartite adaptor: the LRR region recruits partners, while the CH domain anchors to actin-rich structures. This arrangement causally supports GO:0005515 protein binding as the molecular function, with the LRR array supplying multivalent affinity and the CH domain conferring cytoskeletal engagement.

3. 03

   From this molecular toolkit, the biological role follows.

   A CH-anchored scaffold that concentrates partners is well-suited to organize higher-order cytoskeletal systems. The explicit coupling of an LRR assembly platform to an actin-binding CH domain provides a mechanism to position and stabilize septin structures—filamentous assemblies that template actin and membrane organization. This directly aligns with GO:0032185 septin cytoskeleton organization: the LRR array nucleates septin- and membrane-associated complexes, while the CH domain tethers the assembly to actin-rich sites where septins polymerize and cap filaments.

4. 04

   Cellular placement is consistent with soluble cytoskeletal hubs.

   The absence of transmembrane features and the presence of soluble interaction modules point to a cytosolic pool that dynamically associates with filament systems, matching GO:0005829 cytosol and the broader cytoplasmic designation. The experimentally observed cytoplasmic localization is therefore a natural consequence of this soluble adaptor architecture.

5. 05

   Mechanistically, I hypothesize that the LRR block captures septin hetero-oligomers and membrane-curvature proteins, concentrating them into pre-filament assemblies.

   The downstream CH domain then stabilizes these assemblies at actin-dense cortical regions, biasing septin filament nucleation and maintenance. Likely interaction partners include septin family members (to supply filament cores), actin and actin-binding proteins (to provide anchorage and spatial cues), and membrane-associated adaptors that sense curvature—together forming a cytosolic scaffold that choreographs septin cytoskeleton organization.