Intracellular Kinases Overview
Protein kinases are an enzymatically transferred phosphate group from ATP to a serine, threonine or tyrosine residue. Phosphorylation usually leads to the change of enzyme activity, cellular location, or association with other proteins, resulting in functional changes in the target proteins. There are nearly 560 protein kinase genes in human genome and they constitute about 2% of all human genes. Kinase activity is a crucial post-translational modification known to regulate the majority of cellular pathways, especially those involved in signal transduction, including the cell cycle, differentiation, metabolism, and neuronal communication. In addition, abnormal activity of kinase including Akt, ERK, JNK, PKC, PKA, P38, and other MAPK are implicated in many disease states. Intracellular signaling pathways always coordinate with each other to regulate proliferation, differentiation, and apoptosis of cells. The human protein kinase family is divided into the following 7 groups: (1) AGC kinases consist of KA, PKC and PKG; (2) CaM kinases consist of the calcium/calmodulin-dependent protein kinases; (3) CK1 contains the casein kinase 1 group; (4) CMGC kinases consist of CDK, MAPK, GSK3 and CLK kinases; (5) STE includes the homologs of yeast Sterile 7, Sterile 11, and Sterile 20 kinases; (6) TK represents the tyrosine kinases; (7) TKL consists the tyrosine-kinase like group of kinases.
There are also protein kinases including the pseudokinase sub-family (exhibiting unusual features including atypical nucleotide binding and weak, or no, catalytic activity) found in bacteria and plants.
Kinase Structure and Protein Phosphorylation
Protein kinases possess highly conserved catalytic subunits, including a glycine-rich stretch of residues in the N-terminal extremity (involved in ATP binding), a conserved aspartic acid residue in the central part of the catalytic domain (being important for the catalytic activity of the enzyme). The catalytic subunit transfers the phosphate from nucleoside triphosphates (ATP) to amino acid residues in a substrate protein, resulting in a conformational change.
Protein kinases play important roles in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation, thus their activity is highly regulated. Their kinases activity is always turned on or off by phosphorylation mediated by other kinase or itself.
Figure 1. The eukaryotic protein kinase domain Structural elements
Intercellular Signaling and Signal Transduction
Cellular fates including proliferation, differentiation, and apoptosis are dependent upon different combinations of the intracellular signaling pathways, mediated by MAP Kinases (ERK, p38, JNK), AKT, PKA (cAMP), JAK/STATs, and several others. In most cases, many protein kinases are integrated into a cascade mediating a chain of reactions to transmit signals from the cell surface to a variety of cells. It is the process called intracellular signal transduction, by which intracellular signaling pathways connect the cell surface to the nucleus, leading to changes in gene expression in response to extracellular stimuli. The following are some major signaling pathways involved in signal transduction.
In the JAK/STAT pathway, protein-tyrosine phosphorylation directly affects the localization and function of transcription factors, providing immediate connection between protein-tyrosine kinases and transcription factors. STAT proteins are the key elements in this pathway. The inactive STAT proteins are distributed in the cytoplasm. Once stimulated to aggregate, cytokine receptors lead to recruitment of STAT proteins by binding the SH2 domains via their phosphotyrosine-containing sequences in the cytoplasmic domains. Members of the JAK family of non-receptor protein tyrosine kinases associated with cytokine receptors phosphorylate STAT proteins at their tyrosine sites. Tyrosine phosphorylation promotes the dimerization of STAT proteins. Then the STAT protein dimers translocate to the nucleus and stimulate the transcription of their target genes. Recent studies have shown that STAT protein is also a member of activated receptor protein-tyrosine kinases.
The MAP kinase pathway is another important and complex cascade of protein kinase. It is highly conserved in evolution and is ubiquitous in all eukaryotic cells, ranging from yeasts to humans. In higher eukaryotes, activation of MAP kinase pathway promotes cell division and differentiation, and many forms of cancer are associated with aberrations in it. In yeasts, MAP kinase pathways control their mating, cell shape, and sporulation. MAP kinases, a family of protein-serine/threonine kinases are the central elements in the pathway, activated in response to a variety of growth factors and other signaling molecules.
ERK (extracellular signal-regulated kinase) family is the best-characterized forms of MAP kinase in mammalian cells. Its activation plays a central role in cell proliferation. Activation of ERK is mediated by the upstream protein kinases Raf and a second protein kinase called MEK. Raf is coupled to growth factor receptors by Ras, a GTP-binding protein. Activation of Ras leads to activation of the Raf, thereby phosphorylating and activating MEK. Once activated, ERK phosphorylates a variety of targets, including ERK family, other protein kinases and transcription factors.
cAMP pathway works by activating protein kinase A (PKA, cAMP-dependent protein kinase), thereby controlling further effects based on the type of cell. Epinephrine is mediated by the increase of intracellular concentration of cAMP. In this signaling, cAMP is a second messenger activated by the first messenger, hormone itself. In the regulation of glycogen metabolism, protein kinase A phosphorylates another protein kinase. The phosphorylated kinase in turn phosphorylates and activates glycogen phosphorylase, which catalyzes the breakdown of glycogen to glucose-1-phosphate.
Figure 2. Three stages of signal transduction