Attenuated function of a variant form of the helix-loop-helix protein, Id-3, generated by an alternative splicing mechanism. 
The Id family of helix-loop-helix proteins function as negative regulators of DNA binding, basic helix-loop-helix proteins in the regulation of cell growth and differentiation. We report here on the identification of a 17 kDa variant of the 14 kDa Id-3 protein termed Id-3L (long version) which possesses a unique 60 amino acid carboxy-terminus generated by read through of a 'coding intron' and alternative splicing. Northern analysis revealed expression of a minor 1.1 kb Id-3L transcript together with the predominant 0.95 kb Id-3 transcript in the majority of adult human tissues analysed. The variant Id-3L protein is functionally distinguishable from conventional Id-3 since in in vitro DNA mobility shift assays, it was greatly impaired in its ability to abrogate binding of the basic helix-loop-helix protein, E47, to an E box recognition sequence.