Small GTPases

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The members of the protein family of small GTPases (also called small G proteins ) are small proteins that function as molecular “switches” in signal transduction chains through the alternating binding of the nucleotides GDP or GTP . They have a molar mass of 20-25  kDa and are characterized by five conserved structural elements, on the basis of which they can be easily identified and assigned. Based on sequence comparisons, the family of small GTPases can be divided into the subfamilies Ras , Rho , Rab , Sar1 / Arf and Ran . The small G-protein Rop ("Rho of plants"), which owes its name to the homology to the animal Rho, is found in plants.


All small GTPases characterized so far are associated with membranes . The association with the from lipids constructed cell membrane , in most cases, by a post-translationally attached to the N- or C-terminus of the fatty acid radical mediated (farnesyl, geranylgeranyl, myristyl or palmityl radical). In the most common prenylation , farnesyl or geranylgeranyl residues are coupled to a cysteine, which is part of the C-terminal recognition motif CaaX (C: cysteine, a: aliphatic amino acid, X: leucine, methionine, serine or glutamine). In addition to prenylation, some Ras-GTPases have a second post-translational modification in the form of palmitoylation ( e.g. H-Ras and N-Ras). In other cases there are positively charged amino acids in a polybasic region in the C-terminus of the small GTPase, which also support membrane association by interacting with negatively charged groups in the polar part of membrane lipids. The five conserved regions of the small GTPases are necessary for nucleotide and Mg 2+ binding, for intrinsic GTPase activity and for the conformational change induced by the nucleotide exchange.


Small GTPases take on a variety of tasks in the cell: They are involved in the growth and differentiation of cells (e.g. Ras, Ral), regulate the structure of the cytoskeleton and thus cell shape and cell migration (e.g. Rho, Rac, Cdc42, Ral), they are at involved in nuclear import (e.g. Ran) and regulate exocytosis and endocytosis as well as intracellular vesicle transport (e.g. Arf, Rab family, Ral). The best known representative of the small GTPases is the protein Ras. Mutations that constitutively activate Ras play an important role in the development of carcinomas .


GTPase cycle

Small GTPases bind guanine nucleotides with high affinity. Accordingly, the dissociation of the bound nucleotides is very low. The change from GDP to GTP-bound form and thus the activation of the GTPases is therefore catalyzed by so-called "guanine nucleotide exchange factors" ( GEF ). In the meantime, several GEFs have been identified for the same GTPase in several cases. T. are expressed in a tissue- or development-specific manner or activated by different signals and in this way contribute to the specificity of the activation of a GTPase. During the activation of small GTPases by GEFs, various intermediate stages are passed through which, in a comparable manner, also occur during the activation of the Gα subunit of heterotrimeric G proteins. The following mechanism has been proposed for the activation of GTPases by GEFs:

  1. Binding of the GEF with low affinity to the GDP-bound GTPase
  2. Opening of the nucleotide binding site of the GTPase and destabilization of the high affinity GDP bond
  3. Dissociation of the GDP from this initial complex
  4. Formation of a binary, nucleotide-free GEF-GTPase complex in which both proteins interact with each other with high affinity
  5. Uptake of free GTP present in the cell in a concentration about 10 times higher than GDP
  6. Induction of the dissociation of the GTPase-GEF complex due to a conformational change of the GTPase induced by the binding of GTP.

GEFs therefore have two tasks: they destabilize the strong GDP-GTPase interaction and stabilize the nucleotide-free state of the GTPase.

Two variable regions change their conformation depending on the bound nucleotide. These regions are therefore called "switch I" and "switch II" regions. The change in conformation of the GTPase induced by the GTP bond enables interaction with so-called effector molecules via the “switch I” region. By definition, these are proteins that primarily interact with the activated, GTP-bound form of GTPase and are specifically localized and / or activated in this way. The “switch I” region is therefore often called the “effector loop”. The change from GTP to GDP-bound form, i.e. the inactivation of the small GTPases, is catalyzed by so-called "GTPase-activating proteins" ( GAP ), which strengthen the intrinsic GTPase function of the small GTPases and thus hydrolyze the GTP to GDP cause.

For some GTPases such as Rab or Rho-GTPases so-called "guanine nucleotide dissociation inhibitors" (GDI) have been described, which keep the GTPase in the cytoplasm in an inactive state: they stabilize the GDP-bound state of the GTPase and release the GTPase from the Membrane by binding to the prenylated C-terminus of the GTPases, which normally anchors the protein in the membrane. GDI-regulated GTPases are therefore also cytoplasmic as GTPase-GDI complexes. The dissociation of the GTPase-GDI complex is catalyzed by "GDI dissociation factors" (GDF).