The human CD45 gene encodes five isoforms of a transmembrane tyrosine phosphatase that differ in their extracellular domains as a result of alternative splicing of exons 4 -6. Expression of the CD45 isoforms is tightly regulated in peripheral T cells such that resting cells predominantly express the larger CD45 isoforms, encoded by mRNAs containing two or three variable exons. In contrast, activated T cells express CD45 isoforms encoded by mRNAs lacking most or all of the variable exons. We

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... previously identified the sequences within CD45 variable exon 4 that control its level of inclusion into spliced mRNAs. Here we map the splicingregulatory sequences within CD45 variable exons 5 and 6. We show that, like exon 4, exons 5 and 6 each contain an exonic splicing silencer (ESS) and an exonic splicing enhancer (ESE), which together determine the level of exon inclusion in naïve cells. We further demonstrate that the primary activation-responsive silencing motif in exons 5 and 6 is homologous to that in exon 4 and, as in exon 4, binds specifically to the protein heterogeneous nuclear ribonucleoprotein L. Together these studies reveal common themes in the regulation of the CD45 variable exons and provide a mechanistic explanation for the observed physiological expression of CD45 isoforms. . 3 The abbreviations used are: ESE, exonic splicing enhancer; ESS, exonic splicing silencer; RT, reverse transcription; ARS, activation-responsive sequence; PMA, phorbol 12-myristate 13-acetate; FR, -fold repression; hnRNP, heterogeneous nuclear ribonucleoprotein; PTB, polypyrimidine tract-binding protein.