G-protein signaling H-RAS regulation pathway

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 [1].

H-Ras is localized at the cytoplasmic surface of the plasma membrane. It is atarget of posttranslational modification via attachment of farnesyl or methyl lipidmoieties catalyzed by Farnesyltransferase ( FTase ) and Methyltransferase (ICMT ), respectively. These posttranslational modifications affect localizationand biological activity of H-Ras [2], [3].

Like other G-proteins, H-Ras is found in two interconvertible forms, GDP-boundinactive and GTP-bound active [1]. Conversion from GDP-bound form toGTP-bound is catalyzed by guanine nucleotide exchange factor (GEF). Activity of GEF isregulated 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, abinary complex of the small G protein and GEF is formed. Then GEF in this complex isreplaced by GTP resulting in formation of the GTP-bound small G protein [1].

Conversion of GTP-bound form to GDP-bound form is a result of slow intrinsic GTPaseactivity of H-Ras. Proteins known as GTPase activated proteins (GAP) have beenshown to stimulate this reaction. GAPs that inactivate H-Ras are p120GAPand 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 tyrosinekinase activity.

The phosphotyrosines of the receptors, such as platelet-derived growth factor receptorbeta ( PDGF-R-beta ), serve as docking sites for the adaptor proteins, such as Srchomology 2 domain containing transforming protein ( Shc ). Shc forms anadaptor protein complex with Growth factor receptor bound 2 ( GRB2 ). This proteincomplex recruits SOS, the most characterized H-Ras GEF, from the cytosolto produce a receptor-adaptor-GEF complex [4], [5].

G-protein-coupled receptors (GPCRs) can also activate H-Ras signaling. TheBeta-1 adrenergic receptor binds to the PDZ-GEF1 leading to H-Rasactivation [6].

Other receptors, e.g., RET proto-oncogene ( RET ) and TEK tyrosine kinaseendothelial ( TIE2 ), can directly activate Docking proteins 1 and 2 ( DOK1and DOK2 ). DOK1 and DOK2 in turn stimulate the GAP activity ofp120GAP that down-regulate H-Ras signaling [7].

In addition, cytoplasmic Ca(2+) and second messenger 1,2-diacyl-glycerol (DAG ) can activate calcium and DAG-regulated GEFs ( CALDAG-GEF II andCALDAG-GEF III ).

Major effectors of H-Ras protein are protein kinase v-Raf-1 murine leukemiaviral oncogene homolog 1 ( c-Raf-1 ) and Phosphatidylinositol 3-kinase ( PI3Kcat class 1A ) [8], [9], [1], [10].

Small G-proteins are also known to cross-talk with each other. H-Ras activatesguanine nucleotide exchange factors RalRGL and Tiam 1 that in turn activatesmall GTPases RalA and Rac1, respectively [11], [12]. 

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