Abstract
BACKGROUND: NeuroD1 and NeuroD2, members of the bHLH transcription factors family, are key regulators of nervous system development and function. While they share similar roles in neuronal regulation, their divergent expression patterns and specialized functions suggest the complexity of transcriptional control they perform. The activity of bHLH TFs depends on tightly regulated intracellular trafficking orchestrated by NLSs and/or NESs located within the protein's sequences. A detailed characterization of these molecular motifs is essential to understand the mechanisms regulating NeuroD1 and NeuroD2 functions. METHODS: We prepared cDNA vector that enabled expression of the full-length and truncated variants of NeuroD1 and NeuroD2 fused to YFP in COS-7 and N2a cells. Confocal microscopy was used to assess intracellular localization and to identify the location of motifs presenting NLS, NES, and NoLS activity. RESULTS: We demonstrated that previously documented NLS (NLS1), conserved both in NeuroD1 and NeuroD2 also presents NoLS (NoLS1) activity, revealing unexpected dual functionality. Additionally, we identified overlapping NLS2 and NES1 within the bHLH domains of both proteins. Notably, NeuroD2 harbours distinct NLS3 and a second NoLS (NoLS2) motifs, located in the C-terminal region. These elements, suggest differentiated regulation and specialization between the homologs. CONCLUSIONS: Our study reveals a surprisingly complex network of overlapping localization signals in NeuroD1 and NeuroD2 that regulate their cyto-nuclear trafficking. The presence of multiple, potentially competing signals suggests that their activity may be fine-tuned by specific ligands or interacting partners, adding further complexity to their regulation. These findings provide novel insight into how subcellular localization contributes to the functional divergence of homologous transcription factors.