ASCL1 protein domains with distinct functions in neuronal differentiation and subtype specification.

ASCL1 is a neural basic helix-loop-helix (bHLH) transcription factor that plays essential roles during neural development, including neural differentiation and neuronal subtype specification. bHLH factors are defined by their motifs, including a basic region interacting with DNA and an HLH domain involved in protein-protein interactions. We previously defined specific regions within the bHLH domain of ASCL1 as important for its specific functions directing neuronal differentiation in the chick neural tube. Here, we build upon these findings to show how specific mutations within the basic region block DNA binding but not heterodimer formation with E-protein partners TCF3 (E12/E47) and TCF12 (HEB) yet have differential abilities to show dominant negative phenotypes. Additionally, truncating domains outside the bHLH define a nuclear localization signal, a requirement for the C-terminal acidic residues, and the non-essentiality of the N-terminal glutamine/alanine repeats. This structure/function analysis identifies functional domains for ASCL1 activity.