Catalytic domain of STE family Protein Kinases. Protein Kinases (PKs), STE family, catalytic (c) domain. PKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine or tyrosine residues on protein substrates. The STE family is part of a larger superfamily that includes the catalytic domains of other protein serine/threonine kinases (STKs), protein tyrosine kinases (PTKs), RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase (PI3K). This family is composed of STKs, and some dual-specificity PKs that phosphorylate both threonine and tyrosine residues of target proteins. Most members are kinases involved in mitogen-activated protein kinase (MAPK) signaling cascades, acting as MAPK kinases (MAPKKs), MAPK kinase kinases (MAPKKKs), or MAPK kinase kinase kinases (MAP4Ks). The MAPK signaling pathways are important mediators of cellular responses to extracellular signals. The pathways involve a triple kinase core cascade comprising of the MAPK, which is phosphorylated and activated by a MAPKK, which itself is phosphorylated and activated by a MAPKKK. Each MAPK cascade is activated either by a small GTP-binding protein or by an adaptor protein, which transmits the signal either directly to a MAPKKK to start the triple kinase core cascade or indirectly through a mediator kinase, a MAP4K. Other STE family members include p21-activated kinases (PAKs) and class III myosins, among others. PAKs are Rho family GTPase-regulated kinases that serve as important mediators in the function of Cdc42 (cell division cycle 42) and Rac. Class III myosins are motor proteins containing an N-terminal kinase catalytic domain and a C-terminal actin-binding domain, which can phosphorylate several cytoskeletal proteins, conventional myosin regulatory light chains, as well as autophosphorylate the C-terminal motor domain. They play an important role in maintaining the structural integrity of photoreceptor cell microvilli.

Catalytic domain of the Protein Serine/Threonine Kinase, Mitogen-Activated Protein Kinase Kinase Kinase. Serine/threonine kinases (STKs), mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) subfamily, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The MAPKKK subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. MAPKKKs (MKKKs or MAP3Ks) are also called MAP/ERK kinase kinases (MEKKs) in some cases. They phosphorylate and activate MAPK kinases (MAPKKs or MKKs or MAP2Ks), which in turn phosphorylate and activate MAPKs during signaling cascades that are important in mediating cellular responses to extracellular signals. This subfamily is composed of the Apoptosis Signal-regulating Kinases ASK1 (or MAPKKK5) and ASK2 (or MAPKKK6), MEKK1, MEKK2, MEKK3, MEKK4, as well as plant and fungal MAPKKKs. Also included in this subfamily are the cell division control proteins Schizosaccharomyces pombe Cdc7 and Saccharomyces cerevisiae Cdc15.

Protein kinase domain.

Serine/Threonine protein kinases, catalytic domain. Phosphotransferases. Serine or threonine-specific kinase subfamily.

Catalytic domain of Protein Kinases. Protein Kinases (PKs), catalytic (c) domain. PKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine or tyrosine residues on protein substrates. The PK family is part of a larger superfamily that includes the catalytic domains of RIO kinases, aminoglycoside phosphotransferase, choline kinase, phosphoinositide 3-kinase (PI3K), and actin-fragmin kinase. PKs make up a large family of serine/threonine kinases, protein tyrosine kinases (PTKs), and dual-specificity PKs that phosphorylate both serine/threonine and tyrosine residues of target proteins. Majority of protein phosphorylation, about 95%, occurs on serine residues while only 1% occurs on tyrosine residues. Protein phosphorylation is a mechanism by which a wide variety of cellular proteins, such as enzymes and membrane channels, are reversibly regulated in response to certain stimuli. PKs often function as components of signal transduction pathways in which one kinase activates a second kinase, which in turn, may act on other kinases; this sequential action transmits a signal from the cell surface to target proteins, which results in cellular responses. The PK family is one of the largest known protein families with more than 100 homologous yeast enzymes and 550 human proteins. A fraction of PK family members are pseudokinases that lack crucial residues for catalytic activity. The mutiplicity of kinases allows for specific regulation according to substrate, tissue distribution, and cellular localization. PKs regulate many cellular processes including proliferation, division, differentiation, motility, survival, metabolism, cell-cycle progression, cytoskeletal rearrangement, immunity, and neuronal functions. Many kinases are implicated in the development of various human diseases including different types of cancer.