P21 GTPase

Table 4. Reversion by Mg2+ of the inhibition of ras p21 GTPase activity caused by Al3+ ...

GxxGxxKT P-loop Ras-p21-GTPase, PEP carboxykinase, uridylate kinase P-loops occur in many doubly wound O/p structures. This motif functions by binding the phosphate backbone of a mononucleotide. 

Pomerance M, Schweighoffer F, Tocque B, Pierre M. 1992. Stimulation of mitogen-activated protein kinase by oncogenic Ras p21 in Xenopnis oocytes. Requirement for Ras p21-GTPase-activating protein interaction. J Biol Chem 267(23) 16155-16160. 

Subcellular fractionation and immunofluorescence of normal cellular and viral ras proteins have indicated that p21 is localized on the inner face of the plasma membrane. Both the normal and the mutant ras proteins bind GTP specifically and strongly, but only the normal protein has GTPase activity. 

Cepus, V., Scheidig, A. J., Goody, R. S., and Gerwert, K. (1998). Time-resolved FTIR studies of the GTPase reaction of H-ras p21 reveal a key role for the beta-phosphate. Biochemistry 37, 10263-10271. 

Gideon, P., John, J., Freeh, M., Lautwein, A., Clark, R., Scheffler, J. E., and Wittinghofer, A. (1992). Mutational and kinetic analyses of the GTPase-activating protein (GAP) -p21 interaction The C-terminal domain of GAP is not sufficient for full activity. Mol. Cell. Biol. 12, 2050-2056. 

Recent research focus now on applying FTIR methods to proteins without an intrinsic chromophore. In investigating oncogenic hras p21, for instance, protein activity is stimulated with a photolabile substance such as caged GTP (Fig 6.6-21). A UV laser flash, which separates the head group and releases GTP, triggers the GTPase activity of p21 This new experimental approach offers now a very broad applicability to study chemical reactions by time-resolved FTIR. 

Hill CS, Wynne J, Treisman R (1995) The Rho family GTPases RhoA, Racl, and CDC42Hs regulate transcriptional activation by SRF. In Cell 81 1159-70 Hirata K, Kikuchi A, Sasaki T et al. (1992) Involvement of rho p21 in the GTP-enhanced calcium ion sensitivity of smooth muscle contraction. In J. Biol. Chem. 267 8719-22... 

Schweins, T., et al. (1996). Linear free energy relationships in the intrinsic and GTPase activating protein-stimulated guanosine 5 -triphosphate hydrolysis of p21 ras. Biochemistry 35, 14225-14231... 

The normal ras p21 binds GTP and GDP it also hydrolyzes GTP to GDP. The mutant ras p21 proteins show reduced or complete absence of GTPase activity. In this regard, normal p21 appears to be similar to the cellular G (GTP-binding) protein that activates adenylate cyclase and thereby mediates cellular activities. 

More recently it has become clear that also the function of G proteins can be altered. There are a small number of proteins that have been found to associate with G proteins and to affect their function. These proteins include the growth cone associated protein GAP-43, which has been found to enhance GTP binding by Gq in a manner similar to receptors [26], and a complex between the small GTP-binding protein ras p21 and its GTPase activating protein (ras-GAP) which impair coupling of muscarinic receptors to potassium channels [27]. 

Members of this family of small GTPases have members of the family of p21-acti-vated protein kinases (PAKs) as effectors, which phosphorylate and activate MAP3K proteins. Furthermore, the MEKK1 protein has been identifed as a direct effector of GTP-activated Rho/Rac proteins. 

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