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1.  Development of a sensitive non-radioactive protein kinase assay and its application for detecting DYRK activity in Xenopus laevis oocytes 
BMC Biochemistry  2010;11:20.
Although numerous non-radioactive methods are in use to measure the catalytic activity of protein kinases, most require specialized equipment and reagents and are not sufficiently sensitive for the detection of endogenous kinase activity in biological samples. Kinases of the DYRK family have important functions in developmental and pathophysiological processes in eukaryotic organisms including mammals. We aimed to develop a highly sensitive, low-tech assay suitable to determine the activity of DYRK family kinases in tissues or cells from diverse sources.
Phosphorylation-site specific antibodies can be used to monitor the accumulation of the phosphorylated product in kinase assays. We present a modified configuration of an enzyme-linked immunosorbent assay (ELISA)-based kinase assay by using the phosphospecific antibody as the capture antibody. This assay format allowed the detection of small amounts of phosphopeptide in mixtures with an excess of the unphosphorylated substrate peptide (10 fmol phosphorylated peptide over a background of 50 pmol unphosphorylated peptide). Consequently, low substrate turnover rates can be determined. We applied this method to the measurement of endogenous DYRK1A activity in mouse heart tissue by immunocomplex kinase assay. Furthermore, we detected DYRK1-like kinase activity in Xenopus laevis oocytes and identified this kinase as a DYRK1 isoform distinct from the Xenopus DYRK1A ortholog.
We present a non-radioactive and highly sensitive method for the measurement of endogenous activities of DYRKs in biological samples. Xenopus laevis oocytes contain an active DYRK1-related protein kinase more similar to mammalian DYRK1B than DYRK1A.
PMCID: PMC2885986  PMID: 20487523
2.  Mechanism of attenuation of leptin signaling under chronic ligand stimulation 
BMC Biochemistry  2010;11:2.
Leptin is an adipocyte-derived hormone that acts via its hypothalamic receptor (LEPRb) to regulate energy balance. A downstream effect essential for the weight-regulatory action of leptin is the phosphorylation and activation of the latent transcription factor STAT3 by LEPRb-associated Janus kinases (JAKs). Obesity is typically associated with chronically elevated leptin levels and a decreased ability of LEPRb to activate intracellular signal transduction pathways (leptin resistance). Here we have studied the roles of the intracellular tyrosine residues in the negative feedback regulation of LEPRb-signaling under chronic leptin stimulation.
Mutational analysis showed that the presence of either Tyr985 and Tyr1077 in the intracellular domain of LEPRb was sufficient for the attenuation of STAT3 phosphorylation, whereas mutation of both tyrosines rendered LEPRb resistant to feedback regulation. Overexpression and RNA interference-mediated downregulation of suppressor of cytokine signaling 3 (SOCS3) revealed that both Tyr985 and Tyr1077 were capable of supporting the negative modulatory effect of SOCS3 in reporter gene assays. In contrast, the inhibitory effect of SOCS1 was enhanced by the presence of Tyr985 but not Tyr1077. Finally, the reduction of the STAT-phosphorylating activity of the LEPRb complex after 2 h of leptin stimulation was not accompanied by the dephosphorylation or degradation of LEPRb or the receptor-associated JAK molecule, but depended on Tyr985 and/or Tyr1077.
Both Tyr985 and Tyr1077 contribute to the negative regulation of LEPRb signaling. The inhibitory effects of SOCS1 and SOCS3 differ in the dependence on the tyrosine residues in the intracellular domain of LEPRb.
PMCID: PMC2821298  PMID: 20059770
3.  The protein kinase DYRK1A phosphorylates the splicing factor SF3b1/SAP155 at Thr434, a novel in vivo phosphorylation site 
BMC Biochemistry  2006;7:7.
The U2 small nuclear ribonucleoprotein particle (snRNP) component SF3b1/SAP155 is the only spliceosomal protein known to be phosphorylated concomitant with splicing catalysis. DYRK1A is a nuclear protein kinase that has been localized to the splicing factor compartment. Here we describe the identification of DYRK1A as a protein kinase that phosphorylates SF3b1 in vitro and in cultivated cells.
Overexpression of DYRK1A caused a markedly increased phosphorylation of SF3b1 in COS-7 cells as assessed by Western blotting with an antibody specific for phosphorylated Thr-Pro dipeptide motifs. Phosphopeptide mapping of metabolically labelled SF3b1 showed that the majority of the in vivo-phosphopeptides corresponded to sites also phosphorylated by DYRK1A in vitro. Phosphorylation with cyclin E/CDK2, a kinase previously reported to phosphorylate SF3b1, generated a completely different pattern of phosphopeptides. By mass spectrometry and mutational analysis of SF3b1, Thr434 was identified as the major phosphorylation site for DYRK1A. Overexpression of DYRK1A or the related kinase, DYRK1B, resulted in an enhanced phosphorylation of Thr434 in endogenous SF3b1 in COS-7 cells. Downregulation of DYRK1A in HEK293 cells or in HepG2 cells by RNA interference reduced the phosphorylation of Thr434 in SF3b1.
The present data show that the splicing factor SF3b1 is a substrate of the protein kinase DYRK1A and suggest that DYRK1A may be involved in the regulation of pre mRNA-splicing.
PMCID: PMC1420302  PMID: 16512921

Results 1-3 (3)