Ras-GAP protein

It is assumed that the PSD-95 protein functions as an organizer of large protein complexes in the postsynaptic membrane, since other signal proteins such as a NO synthase and a Ras-GAP protein can also specifically bind to the PDZ domain or other domains of PSD-95. In this way, the NMDA receptor is coupled via PSD-95 to other signal proteins in the cell, whereby the NMDA receptor is directly linked to other signal conduction. 

As outlined above, GAP proteins are essential components of signal transmission by Ras. The GAP proteins specific for the Ras proteins are known as Ras-GAP proteins. The importance of Ras-GAP proteins for signal transduction via Ras proteins is shown by oncogenic Ras mutants with amino acid substitutions at positions 12, 13 and 61, which are resistant to the influence of GAP and show constitutive activation of Ras signal transduction. 

The Ras pathway is shown here. Ras is a G-protein that couples signaling from growth factors. The activated receptor is a GNRP that increases the exchange of GDP for GTP and activates the G-protein. Ras GAP inactivates the G-protein. The downstream signal for activated Ras is eventually the mitogen-activated protein kinase pathway (MAPK). 

Ataxia telangiectasia mutated (ATM), poly(ADP ribose) polymerase (PARP), DNA-dependent protein kinase, DNA replication factor C, DNA topoisomerase I, DNA fragmentation factor (DFF)45, inhibitor of caspase-activated DNAse (ICAD), lamins A, Bl, and C TRAF-1, Rafl, Ras, GAP, GDP dissociation inhibitor of Rho family GTPases, phospholipase A2, Statl... 

The lifetime of the active GTP-bound state may be reduced by regulatory GTPase activating proteins. The primary fimction of the GTPase activating proteins (GAP) is to negatively regulate the Ras proteins and Ras-related proteins. 

The rate of GTP hydrolysis in the Ras-GTP complex is very low, but is increased by several orders of magnitude by the corresponding GAP protein. The molecular basis of this stimulation was explained by structural determination of the Ras-GAP complex (Scheffzek et al, 1997). The crystal structure of the complex of a fragment of pl20-GAP (GAP-334 in Fig. 9.5) and Ras GDP AIF3 ended a long discussion on the mechanism of GTPase activation (Fig. 9.5). 

The GAP proteins stimulate GTPase activity of the corresponding Ras protein by an active role in catalysis. This mechanism of GTPase stimulation by GAP protein has largely been explained, thanks to the crystal structure of the Ras GAP transition state analog complex (see 9.2.2). The crystal structure of the transition state complex of a Ras-related protein, the RhoA protein, with the corresponding GAP protein has also shown a similar mechanism of GTPase stimulation. 

Signal transduction between Ras protein and Raf kinase is based on a specific interaction of the two proteins, which can only be performed by the activated, GTP-bound form of Ras protein. The inactive, GDP-form of Ras protein shows significantly weaker binding to Raf kinase (Herrmann et al., 1995). Complex formation is not linked to stimulation of GTPase activity of Ras protein, and thus it is assumed that termination of signal transduction only occins on dissociation of Raf kinase or hydrolysis of the boimd GTP. The lifetime of the GTP state is determined by the GTPase activity of the Ras protein, which itself is subject to regulation by GAP proteins. 

Schussler, P., Grevelding, C.G. and Kunz, W. (1997) Identification of Ras, MAP kinases, and a GAP protein in Schistosoma mansoni by immunoblotting and their putative involvement in male-female interaction. Parasitology 1 1 5, 629-634. 

The pleckstrin-homology domain is named after pleckstrin, where it was found first (pleckstrin is a major platelet protein which is a substrate of protein kinase Pleckstrin and brain spectrin contain two domains, 2 about 120 residues each, with similar sequences in their amino- and carboxy-terminal parts. To date, at least 90 such PH-domain sequences have been described in proteins with very diverse functions, such as spectrin,43 cytosolic protein kinases, phospholipase C isoforms (PLC-P, -y, and the GTPase dynamin,44.45 guanine nucleotide exchange factors for monomeric GTP-bind-ing proteins, the GTPase-acdvating protein for Ras, the Ras-GAP, and cytoskeletal and several other proteins. 

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