A number of growth factors stimulate mitogenesis by interacting with a family
of cell surface receptors which possess an intrinsic, ligand-sensitive,
protein tyrosine kinase activity [1]. These receptor tyrosine kinases (RTK)
all share the same topology: an extracellular ligand-binding domain, a single
transmembrane region and a cytoplasmic kinase domain. However they can be
classified into at least five groups. The class III RTK's are characterized
by the presence of five to seven immunoglobulin-like domains [2] in their
extracellular section. Their kinase domain differs from that of other RTK's by
the insertion of a stretch of 70 to 100 hydrophilic residues in the middle of
this domain. The receptors currently known to belong to class III are:
Platelet-derived growth factor receptor (PDGF-R). PDGF-R exists as a homo-
or heterodimer of two related chains: α and β [3].
Macrophage colony stimulating factor receptor (CSF-1-R) (also known as the
fms oncogene).
Stem cell factor (mast cell growth factor) receptor (also known as the kit
oncogene).
Vascular endothelial growth factor (VEGF) receptors Flt-1 and Flk-1/KDR
[4].
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