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 epsilon-subunit of rat eukaryotic protein-synthesis initiation factor 2B (eIF2Bepsilon), which is inhibited by the GSK3-catalysed phosphorylation of Ser(535). There is evidence that GSK3 is only able to phosphorylate eIF2Bepsilon at Ser(535) if Ser(539) is already phosphorylated by another protein kinase. However, no protein kinases capable of phosphorylating Ser(539) have so far been identified. Here we show that Ser(539) of eIF2Bepsilon, 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 Ser(539) permits GSK3 to phosphorylate Ser(535) in vitro and that eIF2Bepsilon is highly phosphorylated at Ser(539) in vivo. The DYRK isoforms also phosphorylate human microtubule-associated protein tau at Thr(212) in vitro, a residue that is phosphorylated in foetal tau and hyperphosphorylated in filamentous tau from Alzheimer's-disease brain. Phosphorylation of Thr(212) primes tau for phosphorylation by GSK3 at Ser(208) in vitro, suggesting a more general role for DYRK isoforms in priming phosphorylation of GSK3 substrates.