GDNF, c-Src, CalDAG-GEFIII, MAGI-1(BAIAP1), FTase, p120GAP, Tiam 1, TIE2, RASA3, DOK2, CalDAG-GEFII, GRB2, Shc, L-Adrenaline extracellular region, DAG, PI3K cat class IA, PDGF-R-beta, RASGRF2, DOK1, RASGRF1, RasGRP4, Ca('2+) cytosol, RET, PDZ-GEF1, H-Ras, GIPC, GFRalpha1, c-Raf-1, BCR, PDGF-B, ICMT, Lck, RalGDS, Beta-1 adrenergic receptor, SOS, Inositol 1,3,4,5-tetrakisphosphate cytosol, Angiopoietin 1
H-Ras signaling pathway
H-Ras belongs to a family of the small 20-40 kDa GTP-binding proteins (G-proteins) called monomeric G-proteins .
H-Ras is localized at the cytoplasmic surface of the plasma membrane. It is a target of posttranslational modification via attachment of farnesyl or methyl lipid moieties catalyzed by Farnesyltransferase ( FTase ) and Methyltransferase ( ICMT ), respectively. These posttranslational modifications affect localization and biological activity of H-Ras , .
Like other G-proteins, H-Ras is found in two interconvertible forms, GDP-bound inactive and GTP-bound active . Conversion from GDP-bound form to GTP-bound is catalyzed by guanine nucleotide exchange factor (GEF). Activity of GEF is regulated by the upstream signals. GEFs that activate H-Ras are Son of Sevenless ( SOS ), PDZ-GEF1, CALDAG-GEF II and CALDAG-GEF III, RASGRF1, RASGRF2, and RasGRP4.
GEF first interacts with the GDP-bound form and releases bound GDP. As a result, a binary complex of the small G protein and GEF is formed. Then GEF in this complex is replaced by GTP resulting in formation of the GTP-bound small G protein .
Conversion of GTP-bound form to GDP-bound form is a result of slow intrinsic GTPase activity of H-Ras. Proteins known as GTPase activated proteins (GAP) have been shown to stimulate this reaction. GAPs that inactivate H-Ras are p120GAP and RASA3.
The activity of GEPs and GAPs is induced by a large variety of extracellular signals, most notably by those that activate receptors with intrinsic or associated tyrosine kinase activity.
The phosphotyrosines of the receptors, such as platelet-derived growth factor receptor beta ( PDGF-R-beta ), serve as docking sites for the adaptor proteins, such as Src homology 2 domain containing transforming protein ( Shc ). Shc forms an adaptor protein complex with Growth factor receptor bound 2 ( GRB2 ). This protein complex recruits SOS, the most characterized H-Ras GEF, from the cytosol to produce a receptor-adaptor-GEF complex , .
G-protein-coupled receptors (GPCRs) can also activate H-Ras signaling. The Beta-1 adrenergic receptor binds to the PDZ-GEF1 leading to H-Ras activation .
Other receptors, e.g., RET proto-oncogene ( RET ) and TEK tyrosine kinase endothelial ( TIE2 ), can directly activate Docking proteins 1 and 2 ( DOK1 and DOK2 ). DOK1 and DOK2 in turn stimulate the GAP activity of p120GAP that down-regulate H-Ras signaling .
In addition, cytoplasmic Ca(2+) and second messenger 1,2-diacyl-glycerol ( DAG ) can activate calcium and DAG-regulated GEFs ( CALDAG-GEF II and CALDAG-GEF III ).
Small G-proteins are also known to cross-talk with each other. H-Ras activates guanine nucleotide exchange factors RalRGL and Tiam 1 that in turn activate small GTPases RalA and Rac1, respectively , .