A TRANSFORMING GROWTH-FACTOR-BETA REGULABLE RNA-BINDING PROTEIN INTERACTS SPECIFICALLY WITH GERMLINE IG ALPHA-TRANSCRIPTS

Isotype switching is presaged by the transcriptional activation of the heavy chain class gene (CH) to which recombination will occur. As a r esult, mRNA or germline transcripts from the unrearranged gene accumul ate in the cytoplasm. Previous studies demonstrated that transforming growth factor (TGF)-beta stimulated isotype switching to IgA in cultur es of lipopolysaccharide (LPS)-stimulated murine B cells and increased the stability of C-alpha mRNAs. The present study demonstrates that L PS-stimulated B cells express a 45 kDa protein, l alpha BP, that speci fically binds to germline a transcripts. Following addition of TGF-bet a, the binding activity of this protein is significantly reduced. The identification of a cytokine regulable RNA-binding protein that intera cts with germline transcripts supports the idea that these transcripts are involved in recombination and raises the possibility that RNA-pro tein interactions play a role in regulating isotype switching.