EXTENSIVE ALTERNATIVE SPLICING AND DUAL PROMOTER USAGE GENERATE TCF-1PROTEIN ISOFORMS WITH DIFFERENTIAL TRANSCRIPTION CONTROL PROPERTIES

Previously, we reported the isolation of cDNA clones representing four alternative splice forms of TCF-1, a T-cell-specific transcription fa ctor, In the present study, Western blotting (immunoblotting) yielded a multitude of TCF-1 proteins ranging from 25 to 55 kDa, a pattern not simply explained from the known splice alternatives. Subsequent cDNA cloning, PCR amplification, and analysis by rapid amplification of 5' cDNA ends revealed (i) the presence of an alternative upstream promote r, which extended the known N terminus by 116 amino acids, (ii) the pr esence of four alternative exons, and (iii) the existence of a second reading frame in the last exon encoding an extended C terminus. Inclus ion of the extended N terminus into the originally reported protein re sulted in a striking similarity to the lymphoid factor Lef-1. Several of the TCF-1 isoforms, although less potent, mimicked Lef-1 in transac tivating transcription through the T-cell receptor cw-chain (TCR-alpha ) enhancer. These data provide a molecular basis for the complexity of the expressed TCF-1 proteins and establish the existence of functiona l differences between these isoforms. Furthermore, the functional redu ndancy between Tcf-1 and Lef-1 explains the apparently normal TCR-alph a expression in single Tcf-1 or Lef-1 knockout mice despite the firm i n vitro evidence for the importance of the Tcf/Lef site in the TCR-alp ha enhancer.