GSK-3ß (Glycogen Synthase Kinase - 3ß)

Definition
Glycogen synthase kinase 3 (GSK3 ) is a serine/threonine kinase that has enzymatic activity regulated by many signaling pathways and by distinct multiprotein complexes.

Related peptides
GSK3 belongs to the superfamily of mitogen-activated protein (MAP) kinases. Mammalian cells have two glycogen synthase kinase isoforms: and . The protein sequences deviate substantially outside the kinase core (308 residues), which have 97% sequence similarity.

Discovery
GSK-3 was first identified as one of the protein kinases that phosphorylates glycogen synthase, the rate-limiting enzyme of glycogen deposition1,2.. In 1990, Woodgett JR and others, identified the cDNA sequence encoding the 47Kda protein, GSK 3b and co-purified GSK-3a and b, from bovine brain3.

Structural Characteristics
GSK3 has the typical two-domain kinase fold with a -strand domain (residues 25-138) at the N-terminal end and an -helical domain at the C-terminal end (residues 139-343). The ATP-binding site is at the interface of the -helical and -strand domain and is bordered by the glycine-rich loop and the hinge. The activation loop (residues 200-226) runs along the surface of the substrate binding groove. The C-terminal 39 residues (residues 344-382) are outside the core kinase fold and form a small domain that packs against the -helical domain. The -strand domain consists of seven antiparallel -strands: strands 2-6 form a -barrel that is interrupted between strand 4 and 5 by a short helix (residue 96-102) that packs against the -barrel. This helix is conserved in all kinases, and two of its residues play key roles in the catalytic activity of the enzyme. Arg 96 is involved in the alignment of the two domains. Glu 97 is positioned in the active site and forms a salt bridge with Lys 85, a key residue in catalysis. GSK3 has two phosphorylation sites that influence the catalytic activity of the protein. Ser 9 is the phosphorylation site for AKT, and the phosphorylation of this residue inactivates GSK3 . Phosphorylation of Tyr 216, located on the activation loop, increases the catalytic activity.

Mode of action
GSK3 phosphorylates multiple substrates but does not phosphorylate all targets in the same manner and with the same efficiency. The canonical phosphorylation sequence recognized by GSK3 , SXXXS, contains two Ser residues separated by three residues. Multiple copies of this motif can be present in the substrate. Several protein substrates, such as glycogen synthase, eukaryotic initiation factor 2B (eIF2b) and adenomatous polyposis coli protein (APC), are first phosphorylated by a different kinase at the P + 4 serine in the SXXXS motif before GSK3 phosphorylates the serine in the P position. This is called primed phosphorylation and is 100-1000 more efficient than phosphorylation without priming. Glycogen synthase has multiple serines (residues 640, 644, 648 and 652) separated by three residues, and those Ser residues are phosphorylated sequentially by GSK3 from the C-terminal end after Ser 656 has been phosphorylated by casein kinase II. GSK3 uses the phosphorylated serine or threonine at the P + 4 position of the substrate to align of the two domains for optimal catalytic activity4.

Functions
Originally identified for its role in the regulation of glycogen metabolism, it is now known that GSK-3ß plays a key role in cellular processes as diverse as cytoskeletal regulation, cell cycle progression, apoptosis, cell fate and specification, and transcriptional/translational initiation. Therefore, functional kinase activity of GSK-3ß is important in a variety of biological and biochemical processes and altered GSK-3ß activity can contribute to a number of pathological processes including bipolar mood disorder, schizophrenia, heart disease, neurodegeneration, Alzheimer's disease and diabetes mellitus5.

References
1.Embi N, Rylatt DB and Cohen P (1980). Glycogen synthase kinase-3 from rabbit skeletal muscle. Separation from cyclic-AMP-dependent protein kinase and phosphorylase kinase. Eur. J. Biochem., 107, 519–527.
2.Hemmings BA, Yellowless D, Kernohan JC, Cohen P (1982). Purification of Glycogen Synthase Kinase 3 from Rabbit Skeletal Muscle, Eur. J. Biochem., 119, 445-451.
3.Woodgett JR (1990). Molecular cloning and expression of glycogen synthase kinase-3/factor A. EMBO J., 9(8), 2431-8.
4.Haar E, Coll JT, Austen DA, Hsiao HM, Swenson L and Jain J (2001). Structure of GSK3b reveals a primed phosphorylation mechanism. Nature Structural Biology, 8, 593 – 596.
5.Bowley E, Mulvihill E, Howard JC, Pak BJ, Gan BS and O'Gorman DB (2005). A novel mass spectrometry-based assay for GSK-3ß activity. BMC Biochemistry, 6, 29.