It is a great honour to have been asked to write the introduction to this book, which celebrates 25 years of research on glycogen synthase kinase 3 (GSK-3), and to recall the events that led to the identification and characterisation of this fascinating protein kinase, which is involved in regulating so many cellular processes.
In late 1971 I had joined the Department of Biochemistry at the University of Dundee as a junior member of the Faculty after spending two years as a postdoctoral fellow studying the regulation of glycogen metabolism in Edmond's Fischer's laboratory at the University of Washington, Seattle. At that time only four enzymes were known that were regulated by phosphorylation, namely glycogen phosphorylase (activated by phosphorylase kinase) , phosphorylase kinase  and glycogen synthase  (activated and inactivated, respectively, by cyclic AMP-dependent protein kinase, PKA), and pyruvate dehydrogenase (inactivated by pyruvate dehydrogenase kinase) . No protein kinase had been purified to homogeneity or characterised in molecular terms, and so I decided to tackle phosphorylase kinase. This led me to discover that its regulation by PKA involved its phosphorylation at two serine residues, one on the a-subunit and one on the P-subunit of the enzyme. This was unexpected, because the regulation of glycogen phosphorylase involved the phos-phorylation of just one serine residue, and I was intrigued to know whether this was a common phenomenon. Looking for another example of "multisite" phosphorylation, I could see from the literature that glycogen synthase was a potential candidate and, armed with a grant from The British Diabetic Association (today called Diabetes UK), I was able to appoint my first postdoctoral fellow, Hugh Nimmo, to start work on this problem in late 1973.
Quite highly purified preparations of muscle glycogen synthase were always contaminated with traces of PKA, so that glycogen synthase became phos-phorylated if it was incubated with MgATP. In order to suppress this activity Hugh used a potent and specific protein inhibitor of PKA, termed the "protein kinase inhibitor" (PKI) but, to his surprise, PKI only partially suppressed the phosphorylation of glycogen synthase. Moreover, the residual glycogen syn-thase kinase activity that remained even in the presence of PKI could not be PKA, because it phosphorylated glycogen synthase at a site(s) that could not be rendered soluble in trichloroacetic acid by digestion with trypsin. In contrast, the major site(s) phosphorylated by PKA was (were) solubilised by trypsin . These experiments demonstrated that purified preparations of glycogen synthase were contaminated with another protein kinase(s), which we termed glycogen synthase kinase 2 (GSK-2) to distinguish it from PKA (GSK-1).
The next step was to purify GSK-2, but it soon became clear that PKA and GSK-2 were not the only glycogen synthase kinases in muscle. One of these protein kinases was not associated with the glycogen particles to which glycogen synthase was attached and was provisionally called GSK-3 in the Colworth Medal Lecture of the Biochemical Society that I delivered in July
1978 . The partial purification and characterisation of GSK-3 was carried out by Noor Embi, a graduate student from Malaysia (currently the Head of the Malaysian Biotechnology Directorate), while an Australian postdoc Dennis Rylatt identified three of the serine residues on glycogen synthase that are targeted by GSK-3, and found that they were all located in the same tryptic peptide. These papers were submitted for publication at the end of
1979 and published in the European Journal of Biochemistry in 1980 [7,8]. A postdoc Brian Hemmings  and then graduate student Jim Woodgett  later completed the purification of GSK-3 to homogeneity. Jim has subsequently become one of leading researchers on GSK-3 and he has contributed the first chapter of this book.
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