Biochemical and Biophysical Research Communications
Breakthroughs and ViewsStructure and regulation of Kit protein-tyrosine kinase—The stem cell factor receptor☆
Section snippets
Kit—the stem cell factor receptor
Kit is a type III receptor protein-tyrosine kinase [5] (see [6] for a description of type I through IX receptor protein-tyrosine kinases). The type III class also includes the platelet-derived growth factor (PDGF) receptor (α- and β-chains), the macrophage colony-stimulating-factor receptor (CSF-1), and the Fl cytokine receptor (Flt3). Receptor protein-tyrosine kinases all share the same topology: an extracellular ligand-binding domain, a single transmembrane segment, and a cytoplasmic kinase
Stem cell factor and Kit signaling pathways
Binding of SCF to Kit leads to receptor dimerization and activation of protein kinase activity [10]. The receptor becomes autophosphorylated at tyrosine residues during activation; the resulting phosphotyrosine residues serve as docking sites for signal transduction molecules containing SH2 and phosphotyrosine-binding (PTB) domains. Activated Kit also catalyzes the phosphorylation of substrate proteins.
Kit has the potential to participate in multiple signal transduction pathways as a result of
Overview of Kit protein kinase structure
The Kit protein-tyrosine kinase domain has the characteristic bilobed architecture observed in all protein kinases (Fig. 2) [14], [15]. Residues 582–671 make up the small N-terminal lobe of the kinase, and residues 678-953 make up the large C-terminal lobe with a hinge segment between them. The small lobe has a predominantly antiparallel β-sheet structure and is involved in anchoring and orienting ATP. It contains a glycine-rich (GAGAFG) ATP-phosphate-binding loop composed of residues 596–601.
Kit activation by stem cell factor
In the absence of SCF, which is the stimulatory Kit ligand, Kit exists in a monomeric dormant state. The general mechanism for activation of dormant receptor protein-tyrosine kinases involves binding of the appropriate ligand to the extracellular domain of two receptor monomers, bringing them together, and producing a receptor dimer. SCF exists as a non-covalent dimer, and this dimer binds to two Kit monomers thereby promoting Kit dimer formation [7]. For most receptor protein-tyrosine kinases,
Structure of active Kit
Mol et al. [14], [15] determined the structure of an active and inactive conformation of the human Kit intracellular domain. The construct contained a truncated kinase insert. The active form of Kit was obtained by incubating the enzyme with MgATP to initiate the transphosphorylation reaction. Mass spectrographic analysis of the products revealed that Tyr568 and Tyr570 were the first residues to be phosphorylated. Thus, autophosphorylation of residues in the JM domain occurs before that of the
Structure of inactive Kit
The Kit juxtamembrane domain inhibits kinase activity in cis. The JM segment of inactive Kit forms a V-shaped loop that inserts directly into the interface between the small and large lobes of the kinase (Figs. 2B and 3) [15]. This snapshot of the enzyme suggests that the JM domain has the potential to inhibit Kit by displacing the αC-helix, preventing the activation loop from assuming its extended and active conformation, and preventing the movement of the small and large lobes necessary for
Kit mutations and human neoplasms
Gain-of-function mutations occur in a percentage of human neoplasms including mastocytomas (>90%), gastrointestinal stromal tumors (>70%), sinonasal T-cell lymphomas (17%), seminomas/dysgerminomas (9%), and acute myelogenous leukemia (1%) [25]. Furthermore, autocrine or paracrine activation of Kit has been postulated in numerous other human malignancies including ovarian neoplasms and small-cell lung cancer [25], [26]. Furthermore, a large number of human cancers express Kit. Activating Kit
STI-571 targeted therapy of selected human neoplasms
STI-571 (Gleevec, imatinib) is an important and clinically useful protein-tyrosine kinase inhibitor (see [37] for an overview). In contrast to agents such as cisplatin, doxorubicin, and 5-fluorouracil, which are general cytotoxic compounds, STI-571 is a targeted cancer chemotherapeutic drug. This compound inhibits each of the following protein-tyrosine kinases: Abl, Bcr-Abl, Kit, and the PDGF receptor (α and β). Its clinical efficacy was established first in the treatment of chronic myelogenous
STI-571 binding to Kit
Mol et al. [15] determined the structure of the Kit/STI-571 complex by X-ray crystallography. They report that the drug enters the adenine-binding portion of the active site in the cleft between the small and large lobes; a portion of the drug extends into an adjacent hydrophobic pocket. STI-571 interacts with an inactive conformation by binding to the Phe811 “out” [15] or “off” [22] conformation; it is unable to bind to the active “in” or “on” conformation. The drug forms hydrogen bonds with
Epilogue
Protein phosphorylation and dephosphorylation must be stringently regulated both in time and place. Most protein kinases occur physiologically in an inactive or less active basal state. Non-receptor protein kinases such as Abl and Src are maintained in a basal state by inhibitory intramolecular interactions involving SH2 and SH3 domains [4], [37]. Receptor protein kinases require a stimulatory ligand to convert them from a dormant to an active state. The ErbB/HER family of kinases are the
Acknowledgment
I thank Dr. Clifford D. Mol for providing Fig. 2.
References (43)
Src protein-tyrosine kinase structure and regulation
Biochem. Biophys. Res. Commun.
(2004)Signaling by Kit protein-tyrosine kinase—the stem cell factor receptor
Biochem. Biophys. Res. Commun.
(2005)- et al.
Structure of a c-kit product complex reveals the basis for kinase transactivation
J. Biol. Chem.
(2003) - et al.
Structural basis for the autoinhibition and STI-571 inhibition of c-Kit tyrosine kinase
J. Biol. Chem.
(2004) - et al.
The conformational plasticity of protein kinases
Cell
(2002) - et al.
Regulation of protein kinases; controlling activity through activation segment conformation
Mol. Cell
(2004) Protein Explorer: easy yet powerful macromolecular visualization
Trends Biochem. Sci.
(2002)- et al.
The structural basis for autoinhibition of FLT3 by the juxtamembrane domain
Mol. Cell
(2004) - et al.
KIT mutations are common in testicular seminomas
Am. J. Pathol.
(2004) - et al.
Radiotherapy versus single-dose carboplatin in adjuvant treatment of stage I seminoma: a randomised trial
Lancet
(2005)
Constitutively activating mutations of c-kit receptor tyrosine kinase confer factor-independent growth and tumorigenicity of factor-dependent hematopoietic cell lines
Blood
STI-571: an anticancer protein-tyrosine kinase inhibitor
Biochem. Biophys. Res. Commun.
The ErbB/HER receptor protein-tyrosine kinases and cancer
Biochem. Biophys. Res. Commun.
In vitro and in vivo activity of ATP-based kinase inhibitors AP23464 and AP23848 against activation-loop mutants of Kit
Blood
The origins of protein phosphorylation
Nat. Cell Biol.
Protein kinases-the major drug targets of the twenty-first century?
Nat. Rev. Drug Discov.
The protein kinase complement of the human genome
Science
Human proto-oncogene c-kit: a new cell surface receptor tyrosine kinase for an unidentified ligand
EMBO J.
Signaling by receptor tyrosine kinases
Annu. Rev. Biochem.
Crystal structure of human stem cell factor: implication for stem cell factor receptor dimerization and activation
Proc. Natl. Acad. Sci. USA
Signal transduction via the stem cell factor receptor/c-Kit
Cell. Mol. Life Sci.
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Abbreviations: GIST, gastrointestinal stromal tumors; JM, juxtamembrane; PDGF, platelet-derived growth factor; pTyr, phosphotyrosine; PTB, phosphotyrosine binding; SCF, stem cell factor; SH2, Src homology 2; SH3, Src homology 3.