Programs of tissue differentiation are likely controlled by factors regulating gene expression and protein degradation. In muscle, the degradation of the muscle transcription factor MyoD and its inhibitor Id1 occurs via the ubiquitin-proteasome system. E12 and E47, splice products of the E2A gene, interact with MyoD to activate transcription of the muscle program and are also degraded by the ubiquitin-proteasome system (t(1/2) = approximately 6 h). E12 and E47 each contain two regions of basic amino acids, which, when mutated, lead to cytoplasmic accumulation of the proteins. These NLS mutants (E12(NLS), E47(NLS)) are degraded with a half-life similar to the wild-type proteins. In nonmuscle cells, cotransfection of either E12 or E47 with MyoD extended MyoD's half-life from approximately 1 to approximately 4 h. In addition, cotransfection of either E12 or E47 with Id1 led to a marked reduction in Id1's degradation rate from t(1/2) of approximately 1 to approximately 8 h. Furthermore, the cotransfection of NLS deficient mutants of MyoD or Id1 with E12 or E47 resulted in altered intracellular localization of the proteins largely dependent upon the E12 or E47 moiety. Cotransfection of wild-type MyoD or Id1 with NLS deficient mutants of E12 or E47 also led to an altered intracellular localization of MyoD and Id1. These results demonstrate in vivo that E12 and E47 modulate both MyoD and Id1 degradation and may have implications for the physiological regulation of muscle development.