Differential expression of CD45 isoforms is controlled by the combined activity of basal and inducible splicing-regulatory elements in each of the variable exons.
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 have 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.
Tong
Alan
A
Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9038, USA.
Nguyen
Jason
J
Lynch
Kristen W
KW
D017398:Alternative Splicing; D020816:Amino Acid Motifs; D001483:Base Sequence; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004742:Enhancer Elements, Genetic; D005091:Exons; D005786:Gene Expression Regulation; D020868:Gene Silencing; D034664:Heterogeneous-Nuclear Ribonucleoprotein L / Q000737:chemistry; D006801:Humans; D017493:Leukocyte Common Antigens / Q000096:biosynthesis* / Q000737:chemistry*; D008213:Lymphocyte Activation; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009154:Mutation; D011485:Protein Binding; D020033:Protein Isoforms; D017027:Protein Tyrosine Phosphatases / Q000737:chemistry / Q000378:metabolism; D034622:RNA Interference; D012333:RNA, Messenger / Q000737:chemistry / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013601:T-Lymphocytes / Q000378:metabolism; D014466:Ultraviolet Rays
D003432:Cross-Linking Reagents; D034664:Heterogeneous-Nuclear Ribonucleoprotein L; D020033:Protein Isoforms; D012333:RNA, Messenger; D017493:Leukocyte Common Antigens; D017027:Protein Tyrosine Phosphatases