Garg, K.; Sharma, G.; Samaddar, S.; Banerjee, S.

E3 ubiquitin ligases, integral components of the proteasomal degradation cascade, are critical for regulating the cellular proteome via canonical proteasome-mediated protein degradation; however, the non-canonical functions of these ligases in neuronal development are poorly understood. Our study focuses on a special class of E3 ubiquitin ligases known as RNA Binding Ubiquitin Ligases (RBUL) that harbour RNA-binding domains; allowing them to acquire all the properties of RNA-binding proteins (RBPs) and regulate transcriptional or post-transcriptional changes associated with the control of gene expression in cellular phenotypes. We aim to identify one such RUBL in the context of the highly dynamic yet stringently controlled process of neural proliferation and neural fate determination. MEX3B protein is a member of the MEX3 family and a part of the RBUL class of E3 ligases. It is differentially expressed in Neural Progenitor Cells (NPCs) upon differentiation. Mex3b RNA and protein were found to have temporally opposing expression patterns in presence of basic fibroblast growth factor (bFGF), a key signalling protein involved in neuronal proliferation. MEX3B is required for maintenance of the proliferative state of NPCs, whereas, its knockdown promotes transition from proliferative to differentiation state even in presence of bFGF that restricts differentiation. Furthermore, the knockdown of MEX3B protein results in the appearance of morphological hallmarks associated with early stages of neuronal differentiation including increase in neurite length and complexity. MEX3B interacts with the pro-proliferative transcription activator REST and the long non-coding RNA, HOTAIR. The study suggests the existence of a bFGF-dependent, combinatorial axis involving Mex3b, REST and HOTAIR, for the maintenance of NPC proliferative states. MEX3B, containing RNA binding motifs, is a unique E3 ligase that is necessary for bFGF-dependent proliferation. Mex3b protein invokes its non-canonical function of an RNA binding protein to form a tripartite complex with the transcription activator REST and HOTAIR lncRNA to define the proliferative state of NPCs. The study highlights a unique feature of special E3 ligases in neuronal proliferation during brain development that was previously overlooked.