Glycogen synthase kinase‐3 (GSK‐3) is a protein serine kinase implicated in the cellular response to insulin. The enzyme is the mammalian homologue of the zeste‐white3 (shaggy) homeotic gene of Drosophila melanogaster and has been implicated in the regulation of the c‐Jun/AP‐1 transcription factor. In mammals this protein serine kinase is encoded by two related genes termed GSK‐3 alpha and beta. Here, we demonstrate that these two proteins and the fruit fly protein are phosphorylated on tyrosine in vivo. Moreover, GSK‐3 beta activity and function are shown to be dependent on tyrosine phosphorylation. The modified tyrosine residue is conserved in all members of the GSK‐3 family and is equivalent to that required for activity by mitogen‐activated protein (MAP) kinases. However, unlike MAP kinases, GSK‐3 is highly phosphorylated on tyrosine and thus active in resting cells.

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The Drosophila gene product Wingless (Wg) is a secreted glycoprotein and a member of the Wnt gene family. Genetic analysis of Drosophila epidermal development has defined a putative paracrine Wg signalling pathway involving the zeste‐white 3/shaggy (zw3/sgg) gene product. Although putative components of Wg‐ (and by inference Wnt‐) mediated signalling pathways have been identified by genetic analysis, the biochemical significance of most factors remains unproven. Here we show that in mouse 10T1/2 fibroblasts the activity of glycogen synthase kinase‐3 (GSK‐3), the murine homologue of Zw3/Sgg, is inactivated by Wg. This occurs through a signalling pathway that is distinct from insulin‐mediated regulation of GSK‐3 in that Wg signalling to GSK‐3 is insensitive to wortmannin. Additionally, Wg‐induced inactivation of GSK‐3 is sensitive to both the protein kinase C (PKC) inhibitor Ro31–8220 and prolonged pre‐treatment of 10T1/2 fibroblasts with phorbol ester. These findings provide the first biochemical evidence in support of the genetically defined pathway from Wg to Zw3/Sgg, and suggest a previously uncharacterized role for a PKC upstream of GSK‐3/Zw3 during Wnt/Wg signal transduction.

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Extensive in vitro phosphorylation of a purified preparation of control human brain tau consistently produces four rather than, as previously believed, three tau species on SDS-PAGE. The species thus generated are shifted on SDS-PAGE to positions that match those of PHF-tau isolated from Alzheimer's disease brain. A mixture of recombinant human tau isofonns phosphorylated by GSK-3B gave similar results to those obtained with control human brain tau. In vitro phosphorylation of the individual recombinant isoforms by GSK-3/I showed that the four bands of PHF-tau are likely to consist of isoforms 3R,O alone; 4R,O with 3R,29; 4R,29 with 3R,58 and 4R,58 alone.