Small GTPase

Members of the protein family of small GTPases are small proteins, which act through the binding of the nucleotides alternate GDP or GTP as a molecular "switch" in signal transduction. They have a molecular mass of 20-25 kDa and are characterized by five conserved elements, due to which they can be easily identified and assigned. Based on sequence comparisons, the family of small GTPases in the Ras subfamilies, Rho, Rab, Ran Sar1/Arf and can be divided. In plants, there is the small GTPase Rop ( " Rho of plants " ), which owes its name to the homology to animal Rho.

Construction

All previously characterized small GTPases are associated with membranes. The association with the cell membrane is composed of lipids in most cases, is mediated by the post-translationally added to the N -or C- terminus of the fatty acid residue attached ( farnesyl, geranylgeranyl, myristyl or palmityl ). In the most common prenylation farnesyl Geranylgeranylreste or be coupled to a cysteine, which is a component of the C -terminal CaaX motif recognition (C: cysteine ​​, D: aliphatic amino acid, X is leucine, methionine, serine, or glutamine) is. In addition to the prenylation occurs in some Ras GTPases a second post-translational modification in the form of a 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 the membrane association by interaction with negatively charged groups in the polar part of the membrane lipids. The five conserved regions, the small GTPases are necessary for nucleotide and Mg2 binding, the intrinsic GTPase activity and conformational changes induced by the nucleotide.

Function

Small GTPases take over in the cell a variety of tasks: you are in the growth and differentiation of cells involved (eg, Ras, Ral ), regulate the structure of the cytoskeleton and thus cell shape and cell migration (eg Rho, Rac, Cdc42, Ral ), they are most involved in nuclear import ( for example, Ran ) and regulate exocytosis and endocytosis and intracellular vesicle transport ( for example, Arf, Rab family, Ral ). The best-known representatives of small GTPases is the protein Ras. Mutations that activate Ras constitutively, play an important role in the etiology of cancer.

Activation

Small GTPases bind guanine nucleotides with high affinity. Accordingly, the dissociation of the bound nucleotide 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 ). Meanwhile, several GEFs have been identified in several cases for the same GTPase, which in part are expressed in a tissue - or development- specific or activated by different signals and in this way contribute to the specificity of the activation of a GTPase. Upon activation of small GTPases by GEFs various intermediate stages are passed through the heterotrimeric G- proteins occur in a similar manner in the activation of G.alpha subunit. For the activation of GTPases by GEFs following mechanism was proposed:

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

Two variable regions change depending on the bound nucleotide conformation. We call these regions therefore "switch I" - and "switch II " region. -Induced GTP - binding conformation of the GTPase allows using the " switch I " region of the interaction with so-called effector. This is, by definition, proteins that interact primarily with the activated, GTP-bound form of the GTPase and are specifically localized in this way and / or activated. The " switch I " region is therefore also often called " effector loop". The exchange of GTP for GDP- bound form, that is, the inactivation of the small GTPase, is by so-called " GTPase -activating protein " (CAP ) catalyzes which enhance the intrinsic GTPase function of the small GTPase and so the hydrolysis of GTP to GDP cause.

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