Highly Conserved Asparagine in the Basic Domain of Myc Is Dispensable for DNA Binding, Transformation, and Apoptosis

The basic region of c-Myc and other basic helix–loop–helix (b-HLH)-containing proteins bind to the palindromic DNA sequences CANNTG. For the myogenic factor MyoD, a member of the b-HLH family, mutation of several basic region residues abrogates muscle differentiation, but not DNA binding. One of the amino acid positions displaying this behavior in MyoD aligns with a highly conserved asparagine in Myc. This conserved asparagine displays complete tolerance for alanine substitution as measured by DNA binding. To test the possibility of whether the basic region of Myc encodes a second biological function, the conserved asparagine in c-Myc (N360) was mutated to alanine and tested for the Myc-dependent functions of cellular transformation and apoptosis. In contrast to the deleterious effects of such mutations in MyoD, the alanine mutant functions normally for both Myc-dependent cellular transformation and apoptosis induction. Therefore, a basic region function distinct from DNA binding may not be a general feature of HLH transcription factors.