Developmentally Regulated RNA-binding Protein 1 (Drb1)/RNA-binding Motif Protein 45 (RBM45), a Nuclear-Cytoplasmic Trafficking Protein, Forms TAR DNA-binding Protein 43 (TDP-43)-mediated Cytoplasmic Aggregates

Cytoplasmic protein aggregates are one of the pathological hallmarks of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FT LD). Several RNA-binding proteins have been identified as components of inclusion bodies. Developmentally regulated RNA-binding protein 1 (Drbl)/RNA-binding motif protein 45 is an RNA-binding protein that was recently described as a component in ALS- and FTLD-related inclusion bodies. However, the molecular mechanism underlying cytoplasmic Drbl aggregation remains unclear. Here, using an in vitro cellular model, we demonstrated that Drbl co-localizes with cytoplasmic aggregates mediated by TAR DNA-binding protein 43, a major component of ALS and FTLD-related inclusion bodies. We also defined the domains involved in the subcellular localization of Drbl to clarify the role of Drbl in the formation of cytoplasmic aggregates in ALS and FTLD. Drbl predominantly localized in the nucleus via a classical nuclear localization signal in its carboxyl terminus and is a shuttling protein between the nucleus and cytoplasm. Furthermore, we identify a double leucine motif serving as a nuclear export signal. The Drbl mutant, presenting mutations in both nuclear localization signal and nuclear export signal, is prone to aggregate in the cytoplasm. The mutant Drb 1-induced cytoplasmic aggregates not only recruit TAR DNA-binding protein 43 but also decrease the mitochondrial membrane potential. Taken together, these results indicate that perturbation of Drbl nuclear cytoplasmic trafficking induces toxic cytoplasmic aggregates, suggesting that mislocalization of Drbl is involved in the cause of cytotoxicity in neuronal cells.