Phosphate tightly associated with a molecule, e.g., a protein, has been known since the late nineteenth century. Since then, a variety of covalent linkages of phosphate to proteins have been found. The most common involve esterification of phosphate to serine, threonine, and tyrosine with smaller amounts being linked to lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine. The occurrence of phosphorylated molecules, e.g., proteins, implies the existence of one or more kinases, e.g., protein kinases, capable of phosphorylating various molecules, e.g., amino acid residues on proteins, and also of phosphatases, e.g., protein phosphatases, capable of hydrolyzing various phosphorylated molecules, e.g., phosphorylated amino acid residues on proteins.
Protein kinases play critical roles in the regulation of biochemical and morphological changes associated with cellular growth and division (D'Urso et al. (1990) Science 250:786-791; Birchmeier et al. (1993) Bioessays 15:185-189). For example, these kinases have been shown to participate in the transmission of signals from growth-factor receptors (Sturgill et al. (1988) Nature 344:715-718; Gomez et al. (1991) Nature 353:170-173), control of entry of cells into mitosis (Nurse (1990) Nature 344:503-508; Maller (1991) Curr. Opin. Cell Biol. 3:269-275), and regulation of actin bundling (Husain-Chishti et al. (1988) Nature 334:718-721). Protein kinases serve as growth factor receptors and signal transducers and have been implicated in cellular transformation and malignancy (Hunter et al. (1992) Cell 70:375-387; Posada et al. (1992) Mol. Biol. Cell 3:583-592; Hunter et al. (1994) Cell 79:573-582). Alterations in kinase genes and their products can lead to deregulated cell proliferation, a hallmark of cancer. Modulation of these genes and their regulatory activities may permit the control of tumor cell proliferation and invasion.
Protein kinases can be divided into different groups based on either amino acid sequence similarity or specificity for either serine/threonine or tyrosine residues. A small number of dual-specificity kinases have also been described. Within the broad classification, kinases can be further subdivided into families whose members share a higher degree of catalytic domain amino acid sequence identity and also have similar biochemical properties. Most protein kinase family members also share structural features outside the kinase catalytic domain that reflect their particular cellular roles. These include regulatory domains that control kinase activity or interaction with other proteins (Hanks et al. (1988) Science 241:42-52).
Extracellular signal-regulated kinases/mitogen-activated protein kinases (ERKs\MAPKs) and cyclin-directed kinases (Cdks) represent two large families of serine-threonine kinases (see Songyang et al. (1996) Mol. Cell. Biol. 16: 6486-6493). Both types of kinases function in cell growth, cell division, and cell differentiation in response to extracellular stimuli. The ERK\MAPK family members are critical participants in intracellular signaling pathways. Upstream activators as well as the ERK\MAPK components are phosphorylated following contact of cells with growth factors or hormones or in response to cellular stressors, for example, heat, ultraviolet light, and inflammatory cytokines. These kinases transport messages that have been relayed from the plasma membrane to the cytoplasm by upstream kinases into the nucleus where they phosphorylate transcription factors and effect gene transcription modulation (Karin et al. (1995) Curr. Biol. 5: 747-757). Substrates of the ERK\MAPK family include c-fos, c-jun, APF2, and ETS family members Elk1, Sap1a, and c-Ets-1 (cited in Brott et al. (1998) Proc. Natl. Acad. Sci. USA 95: 963-968).
Signal-transduction pathways that employ members of the ERK/MAPK family of serine/threonine kinases are widely conserved among eukaryotes. The multiplicity of these pathways allows the cell to respond to divergent extracellular stimuli by initiating a broad array of responses ranging from cell growth to apoptosis. ERK/MAPK pathways are comprised of a three-tiered core-signaling module wherein ERK/MAPKs are regulated by MAPK/ERK kinases (MEKs), and MEKs, in turn, are regulated by MAPK kinase kinases (MAPKKKs). Mammalian stress-activated ERK/MAPK pathways have been implicated in numerous important physiological functions, including cell growth and proliferation, inflammatory responses, and apoptosis. For example, activation of the ERK1,2 signaling pathway by a mitogenic growth factor, a tumor promoter, or by transformation suppresses decorin gene expression in fibroblasts, which in turn may promote proliferation and migration of normal and malignant cells (Laine et al. (2000) Biochem. J. 349: 19-25).
Cdks regulate transitions between successive stages of the cell cycle. The activity of these molecules is controlled by phosphorylation events and by association with cyclin. Cdk activity is negatively regulated by the association of small inhibitory molecules (Dynlacht (1997) Nature 389:148-152). Cdk targets include various transcriptional activators such as p110Rb, p107, and transcription factors, such as p53, E2F, and RNA polymerase II, as well as various cytoskeletal proteins and cytoplasmic signaling proteins (cited in Brott et al. (1998) Proc. Natl. Acad. Sci. USA 95: 963-968).
Protein kinases play critical roles in cellular growth, particularly in the transduction of signals for cell proliferation, differentiation, and apoptosis. Therefore, novel protein kinase polynucleotides and proteins are useful for modulating cellular growth, differentiation, and/or development.