Combinatorial Regulation of Signal-Induced CD45 Exon Repression by hnRNP L and PSF

CD45 is a hematopoetic-specific tyrosine phoshatase. In resting T cells three variable exons are partially repressed, and following antigen challenge, these exons are more highly repressed. Previous work identified the ESS1 silencer element that functions to mediate exon 4 silencing under resting conditions by binding to hnRNP L. ESS1 is also sufficient to confer the activation-induced increase in exon repression, and this document describes two mechanisms responsible for mediating this effect. First, hnRNP L silencing function is slightly increased in activated cells as compared to resting cells. Additionally, PSF binds to the ESS1 complex in a signal-dependent manner and provides a significant increase in repressive activity. Further investigation shows these two mechanisms are largely independent but show some functional crosstalk, and while neither of these mechanisms is sufficient in isolation, the combination of these two effects accounts for an increase in exon silencing that is of similar magnitude to the total observed change in splicing in response to cellular activation.