Posttranscriptional regulation of Id1 activity in cardiac muscle. Alternative splicing of novel Id1 transcript permits homodimerization

The transcriptional regulatory protein Id, a negatively trans-acting protein with a helix-loop-helix motif that is expressed in many proliferating tissues early in development, continues to be expressed in postmitotic adult cardiac myocytes and vascular smooth muscle. Following the observation of a "doublet" band of 1.1 and 1.25 kilobases on Northern hybridizations of Id1 cDNA with mRNA isolated from both cardiac muscle and vascular smooth muscle cells, we identified and sequenced an alternatively spliced Id1 gene product containing an insert of 214 base pairs within the coding domain of the original Id1 cDNA. A protein with a molecular mass corresponding to that predicted by the Id1.25-kilobase mRNA sequence could be identified on immunoblots of cell lysates from neonatal and adult rat ventricular myocytes. The insert appears to be a "coding intron," based on the presence of intron-exon consensus sequences at the insert boundaries and the presence of the originally described Id1 carboxyl-terminal coding sequence immediately downstream from, and out of frame with, this insert. In contrast to Id1 and Id2, which do not form homodimers, the carboxyl-terminal sequence of this alternatively spliced Id1 transcript, termed Id1.25, permits homodimerization. Thus, alternative splicing of Id1 may allow for tissue-specific expression of Id1, while formation of homodimers could provide a post-translational mechanism to regulate the ability of Id1.25 to bind and inactivate E2A gene products.