Actin associates with bHLH proneural proteins in nucleus and positively regulates neural precursor gene expression. Yun-Ling Hsiao^1,2, Yu-Ju Chen^1,2, Hsiao-Fong Yeh², Haiwei Pi^1,2. 1) Graduate Institute of Biomedical Sciences, College of Medicine, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan; 2) Department of Biomedical Sciences, College of Medicine, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan.

   Proneural proteins of basic helix-loop-helix (bHLH) transcription factors play a central role in neurogenesis. However, little is known about how proneural proteins interact with the transcriptional machinery to activate downstream target gene expression. Drosophila proneural proteins Achaete (Ac) and Scute (Sc) induce external sensory (ES) organ formation. Through co-immunoprecipitation and mass spectormetric (MS) analyses, we found that nuclear actin but not cytoplsmic actin associated with proneural proteins Ac and Sc in Drosophila S2 cells. Daughterless, the Drosophila homologue of E12 and E47 proteins, also associated with nuclear actin through heterodimerization with Ac. Overexpression of NLS-actin that localized predominantly in nuclei elevated proneural protein downstream target gene expression in S2 cells in a Ac/Da-dependent manner. Reduction in actin and ac sc gene activity resulted in a synergistic loss of some ES organs, and decrease in actin5C and actin42A expression level reduced phyllopod transcription in neural precursors. Taken together, our analyses suggest an actin-mediated regulation of bHLH proneural activity in promoting gene expression and precursor specification.