PIPSL

Model summary

A soluble human adaptor that organizes proteostasis and signaling assemblies. Its N-terminal kinase-like scaffold and central adhesion module create multivalent interfaces that bind proteins and RNA, while a C-terminal adaptor module couples ubiquitinated substrates to the proteasome. By coordinating proteasome-mediated degradation with autophagy and growth-factor pathways, it modulates cytosolic and nucleoplasmic quality-control circuits and tunes phosphoinositide-linked signaling.

Predicted Gene Ontology terms

1. 01

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

   The N-terminal half is dominated by a phosphatidylinositol phosphate kinase scaffold: IPR027484 (Phosphatidylinositol-4-phosphate 5-kinase, N-terminal homologous superfamily, residues 26–193) precedes and feeds into IPR027483 (Phosphatidylinositol-4-phosphate 4/5-kinase, C-terminal domain superfamily, residues 194–411). These two superfamily signatures flank and integrate with IPR002498 (Phosphatidylinositol-4-phosphate 4/5-kinase, core domain, residues 28–397) and the family-level IPR023610 (Phosphatidylinositol-4/5-phosphate 5/4-kinase family, residues 14–398). Together, this contiguous N-to-mid region forms a bilobal catalytic-like core typical of lipid kinase folds that bind ATP and acidic phospholipids. In this protein, that fold causes high-affinity nucleotide and membrane-lipid engagement and provides a versatile platform for macromolecular assembly rather than obligate lipid phosphorylation chemistry.

2. 02

   Downstream, the mid-to-C-terminal region introduces interaction-rich modules.

   IPR036465 (von Willebrand factor A-like domain superfamily, residues 488–670) and its embedded IPR002035 (von Willebrand factor, type A domain, residues 490–673) create a metal-ion–dependent adhesion module that commonly mediates avid protein–protein and protein–RNA contacts. At the extreme C-terminus sits IPR049590 (PSMD4, RAZUL domain, residues 805–848), a hallmark of proteasome-associated adaptors that bind ubiquitin and shuttle substrates to the 26S proteasome. The juxtaposition of a kinase-like scaffold, a vWA adhesion domain, and a RAZUL adaptor module yields a modular assembly platform specialized for selective protein and RNA binding and for coupling ubiquitinated cargo to the proteasome.

3. 03

   This architecture dictates molecular function.

   The bilobal PI-kinase-like core and vWA domain together generate broad multivalent interfaces that drive GO:0005515 protein binding. The presence of RNA-associated surfaces within vWA-containing assemblies and the propensity of such scaffolds to engage ribonucleoprotein complexes support GO:0003723 RNA binding. The RAZUL domain confers high-affinity recognition of ubiquitin/proteasome components and positions the protein as a cofactor that promotes ubiquitin transfer reactions; functionally, this aligns with GO:0061630 ubiquitin protein ligase activity, where the protein acts as an E3-associated adaptor that enhances ubiquitin ligation and substrate handoff.

4. 04

   From these molecular activities, the biological processes follow.

   By docking ubiquitinated substrates to the proteasome via the RAZUL module and by recruiting E3 machinery through its kinase-like and vWA platforms, the protein directly drives GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process. Its proteasome-coupling role extends to autophagy pathways: proteasome–autophagy crosstalk and RNA/protein quality-control signaling rationalize GO:0010506 regulation of autophagy. The PI-kinase-like scaffold and vWA-mediated assemblies can tune phosphoinositide-sensitive signaling hubs; by organizing membrane-proximal adaptors and RNA–protein assemblies that feed into growth-factor pathways, the protein modulates GO:0014066 regulation of phosphatidylinositol 3-kinase signaling.

5. 05

   Cellular location is consistent with soluble, assembly-prone domains and absence of transmembrane segments.

   The kinase-like core and vWA/RAZUL modules favor cytosolic and nuclear soluble environments, matching GO:0005829 cytosol and GO:0005654 nucleoplasm. This soluble distribution enables rapid exchange between proteasome-rich cytosolic pools and nuclear proteostasis machinery.

6. 06

   Mechanistically, I propose that the N-terminal kinase-like core nucleates a ribonucleoprotein–rich assembly that senses ATP and acidic phospholipids to stabilize higher-order complexes.

   The vWA domain then strengthens avidity for client proteins and possibly RNA, while the C-terminal RAZUL domain captures ubiquitin and proteasome receptors to channel substrates into degradation. Through this tri-modular cascade, the protein acts as a proteasome-associated adaptor that tunes PI3K/Autophagy signaling and proteasome throughput.

7. 07

   Likely interaction partners follow from this mechanism.

   The RAZUL domain predicts association with 26S proteasome subunits and ubiquitin receptors; the vWA and kinase-like cores suggest contacts with E3 ligases and RNA-binding factors. I therefore hypothesize interactions with proteasome regulatory particle components and ubiquitin shuttle factors, as well as RNA-binding assemblies that coordinate cytosolic and nucleoplasmic proteostasis.