The E3 ligase MEX3B forms a tripartite complex with Rest and Hotair to determine the proliferative capacity of neural progenitor cells.

E3 ubiquitin ligases regulate the cellular proteome via proteasome-dependent protein degradation; however, there exist limited studies outlining their non-canonical functions. RNA-binding ubiquitin ligases (RBULs) represent a subset of E3 ligases that harbour RNA-binding domains, making them uniquely positioned to function as both RNA-binding proteins and E3 ligases. Our initial microarray screen for E3 ligases from mouse cortical neural progenitor cells identified MEX3B, a known RNA-binding ubiquitin ligase, to be differentially expressed. Here, we characterize the non-canonical role of MEX3B in the context of neural proliferation. We find that MEX3B is significantly reduced following the differentiation of neural progenitor cells (NPCs). The knockdown of MEX3B blocks the proliferative state of NPCs and leads to the enhancement of neurite length and dendrite branching. We observed that MEX3B regulates the stability of Rest mRNA in proliferative NPCs. Mechanistically, MEX3B interacts with Rest mRNA and the lncRNA Hotair to form a tripartite complex in the presence of basic fibroblast growth factor (bFGF). Loss of Hotair disrupts this complex; conversely, MEX3B RNAi significantly reduces Hotair abundance. Rest mRNA levels remain unaffected by Hotair knockdown, suggesting that the latter acts as a scaffold to facilitate bFGF-dependent MEX3B-Rest interaction in the MEX3B-Rest-Hotair tripartite axis. Our study demonstrates an RNA-driven post-transcriptional mechanism underlying NPC proliferation.