Abstract
The dual specificity tyrosine phosphorylated and regulated kinase (DYRK) family of protein kinases is a group of evolutionarily conserved protein kinases that have been characterized as regulators of growth and development in mammals, Drosophila and lower eukaryotes. In the present study, we have characterized three splicing variants of DYRK1B (DYRK1B-p65, DYRK1B-p69 and DYRK1B-p75) with different expression patterns and enzymic activities. DYRK1B-p65 and DYRK1B-p69 exhibited similar, but not identical, patterns of expression in mouse tissues, with the highest protein levels found in the spleen, lung, brain, bladder, stomach and testis. In contrast, DYRK1B-p75 was detected specifically in skeletal muscles, in the neuronal cell line GT1-7 and also in differentiated, adipocyte-like 3T3-L1 cells, but not in undifferentiated 3T3-L1 preadipocytes. A comparison of the mouse and human Dyrk1b genomic and cDNA sequences defined the alternative splicing events that produce the variants of DYRK1B. In DYRK1B-p75, transcription starts with exon 1B instead of exon 1A, generating a new translation start, which extends the open reading frame by 60 codons. This gene structure suggests that alternative promoters direct the expression of DYRK1B-p69 and DYRK1B-p75. Both splicing variants exhibited kinase activity in vitro and contained phosphotyrosine when expressed in COS-7 cells. Owing to differential recognition of the 3'-splice site in exon 9, DYRK1B-p65 differs from DYRK1B-p69 by the absence of 40 amino acids within the catalytic domain. DYRK1B-p65 lacked kinase activity in vitro and did not contain phosphotyrosine. DYRK1B-p69 and DYRK1B-p75 stimulated reporter gene activity driven by the f or kh ead in r habdosarcoma (FKHR)-dependent glucose-6-phosphatase promoter more strongly when compared with DYRK1B-p65, indicating that the DYRK1B splicing variants exhibit functional differences.