The substrate specificity of glycogen synthase kinase 3 (GSK3) is unusual in that efficient phosphorylation only occurs if another phosphoserine or phosphothreonine residue is already present four residues C-terminal to the site of GSK3 phosphorylation. One such substrate is the ε-subunit of rat eukaryotic protein-synthesis initiation factor 2B (eIF2Bε), which is inhibited by the GSK3-catalysed phosphorylation of Ser535. There is evidence that GSK3 is only able to phosphorylate eIF2Bε at Ser535 if Ser539 is already phosphorylated by another protein kinase. However, no protein kinases capable of phosphorylating Ser539 have so far been identified. Here we show that Ser539 of eIF2Bε, which is followed by proline, is phosphorylated specifically by two isoforms of dual-specificity tyrosine phosphorylated and regulated kinase (DYRK2 and DYRK1A), but only weakly or not at all by other ‘proline-directed’ protein kinases tested. We also establish that phosphorylation of Ser539 permits GSK3 to phosphorylate Ser535in vitro and that eIF2Bε is highly phosphorylated at Ser539in vivo. The DYRK isoforms also phosphorylate human microtubule-associated protein tau at Thr212in vitro, a residue that is phosphorylated in foetal tau and hyperphosphorylated in filamentous tau from Alzheimer's-disease brain. Phosphorylation of Thr212 primes tau for phosphorylation by GSK3 at Ser208in vitro, suggesting a more general role for DYRK isoforms in priming phosphorylation of GSK3 substrates.

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