LRRIQ4

Model summary

A human LRR-based scaffold that organizes signaling assemblies controlling cell-cycle progression and apoptosis. Its extended leucine-rich repeat solenoid creates a high-affinity platform for multivalent protein binding, enabling assembly and potentiation of Ras/MAPK signaling at membrane-proximal sites. The scaffold shuttles between cytoplasm and nucleus to couple membrane-associated signaling with transcriptional and apoptotic programs and is additionally recruited to photoreceptor disc membranes, where it stabilizes specialized membrane-associated complexes.

Predicted Gene Ontology terms

1. 01

   I begin by dissecting the InterPro architecture, which is dominated by tandem leucine-rich repeat (LRR) modules that extend across the length of the protein.

   The N-terminal half is covered by IPR032675 (Leucine-rich repeat domain superfamily, residues 24–283), followed contiguously by IPR032675 annotations at residues 284–382 and 383–501, indicating a continuous LRR solenoid that likely wraps around a central axis to create a versatile protein–protein interaction surface. Multiple nested LRR annotations reinforce this scaffold: IPR050216 (Leucine-rich repeat domain-containing protein family, residues 235–397) and IPR055414 (Disease resistance R13L4/SHOC-2-like, LRR domain, residues 301–405) specify a SHOC-2–like LRR fold known to organize Ras/MAPK pathway assemblies. A dense array of typical LRR repeats is mapped by IPR003591 and IPR001611 entries across residues 47–466, including [47–69], [70–92], [93–116], [117–140], [141–164], [187–209], [210–233], [234–255], [256–279], [302–324], [325–347], [301–405], [348–371], [397–419], and [420–443]. This ordered, modular repeat architecture causes high-affinity, multivalent binding to partner proteins rather than catalysis, directly supporting GO:0005515 protein binding as the molecular function.

2. 02

   The SHOC-2–like LRR core (IPR055414) is a hallmark scaffold for Ras/MAPK pathway assembly.

   By stabilizing small GTPase effectors and their downstream kinases, this solenoid architecture promotes assembly and signal throughput of Ras-dependent cascades. Such a scaffolded assembly explains positive modulation of small GTPase signaling, aligning with GO:0046579 positive regulation of Ras protein signal transduction. The same binding-centric architecture readily explains roles in cell-cycle control and apoptosis: by tuning Ras/MAPK and allied pathways, the scaffold can bias transcriptional and post-transcriptional programs that govern proliferation and survival.

3. 03

   Cellular placement follows from both the LRR scaffold’s membrane-proximal behavior and the observed compartmentalization.

   The LRR solenoid often associates with membrane-proximal complexes, consistent with GO:0005886 plasma membrane. In specialized sensory neurons, the same scaffold can be recruited to disc membranes of photoreceptors, consistent with GO:0097381 photoreceptor disc membrane. The experimentally observed distribution between cytoplasm and nucleus fits a shuttle-like scaffold that assembles signaling complexes in the cytoplasm and influences transcriptional/apoptotic programs in the nucleus.

4. 04

   Mechanistically, I propose that the LRR array nucleates a Ras/MAPK signalosome at membrane-proximal sites, enhancing pathway fidelity and amplitude.

   This assembly then communicates with nuclear regulators to coordinate cell-cycle progression and apoptosis. In photoreceptors, the scaffold likely stabilizes membrane-associated complexes at disc rims, integrating cargo trafficking and membrane remodeling with signaling status.

5. 05

   From this model, likely interaction partners include small GTPases and their effectors (e.g., HRAS/KRAS/NRAS and RAF kinases), MAPK cascade components (MEK and ERK), SHOC2-family scaffolds, and membrane-associated adaptors at photoreceptor disc membranes.

   These interactions collectively explain the observed protein-binding function, the positive regulation of Ras signaling, and the dual cytoplasm–nucleus localization.