Identification of a human cDNA encoding a novel Bcl-x isoform

Alternative splicing has been shown to generate two isoforms of the apoptosis regulator bcl-x, Bcl-x(L) and Bcl-x(S), in humans. Here we describe the identification and characterization of a third splice variant of the human bcl-x gene. It differs from previously described bcl-x transcripts in two respects: (1) a novel facultative intron is spliced out at the 5' untranslated region and (2) the open reading frame arises from a continuous genomic sequence extending over the splice donor sites utilized by the bcl-x(L) and bcl-x(S) transcripts. Since the resulting molecule has an organisation homologous to mouse and rat Bcl-x(beta) we suggest calling this novel protein human Bcl-x(beta). Northern blot analysis revealed that bcl-x(beta) mRNA is expressed in numerous cell lines. Like Bcl-x(L), h-Bcl-x(beta) binds to the pro-apoptotic protein Lax, suggesting a functional activity in vivo. (C) 1998 Academic Press.