Degradation of Id proteins by the ubiquitin-proteasome pathway

Id proteins act as negative regulators of bHLH transcription factors by for ming; transcriptionally inactive protein complexes. The proposed function o f these proteins includes promotion of cell growth and cell cycle progressi on, induction of apoptosis, and inhibition of cellular differentiation. We investigated the role of the ubiquitin-mediated proteolytic pathway in the degradation of the Id3 protein. We found Id3 to be a short-lived protein an d estimated the half-life to be similar to 20 min in 293 cells, Using speci fic inhibitors of the 26S proteasome and mutant fibroblast cells with a tem perature-sensitive defect in the essential El ubiquitin-activating enzyme, we show that Id3 and the related Idl and Id2 proteins are degraded through the ubiquitin-proteasome pathway. We found the Id4 protein to be much less sensitive to inhibitors of the 26S proteasome, but its degradation was depe ndent on the El enzyme. In addition, we observed that coexpression of the b HLH protein E47 with Id3 significantly reduced the rate of degradation of I d3, suggesting that Id3 is less susceptible to degradation by the 26S prote asome when complexed to a bHLH protein.