The kinase DYRK phosphorylates protein-synthesis initiation factor elF2Bε at Ser⁵³⁹ and the microtubule-associated protein tau at Thr²¹²: Potential role for DYRK as a glycogen synthase kinase 3-priming kinase

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 Ser⁵³⁵ in vitro and that eIF2Bε is highly phosphorylated at Ser⁵³⁹ in vivo. The DYRK isoforms also phosphorylate human microtubule-associated protein tau at Thr²¹² in 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 Ser²⁰⁸ in vitro, suggesting a more general role for DYRK isoforms in priming phosphorylation of GSK3 substrates.