Small GTPases

Similar to Ras and Rho proteins, Rab proteins are posttranslationally modified at the C-terminus with prenyl (geranylgeranyl) groups that function as membrane anchors. Protein prenylation involves covalent attachment of the farnesyl (C-15 isoprenyl) or the geranylgeranyl (C-20 isoprenyl) moiety to one or two C-terminal cysteine residues of the protein substrate via a stable thioether linkage. 

Prenylated small GTPases represent major hubs in most membrane-connected signaling networks. Rab prenylation is mediated by Rab geranylgeranyl transferase (RabGGTase), which works together with an adaptor protein, Rab escort protein (REP). Rab cycling between membranes and the cytosol is made possible by interaction with the GDP dissociation inhibitor (GDI). Both GDI and REP function as molecular chaperones, which can solubilize the prenylated Rab proteins in the cytosol. 

Rab proteins compromise the largest subgroup of the Ras superfamily of small GTPases, with more than 60 members in humans. Each of the Rab GTPases in human cells regulates intracellular vesicular transport at a specific subcellular membrane. While the mechanistic basis for GDI-mediated extraction of Rab molecules from membrane is well understood [7], a thermodynamic model of GDI-mediated membrane delivery of Rab proteins remains to be established. 

Study of the membrane trafficking process regulated by Rab requires prenylated Rab proteins. Previously, there were substantial difficulties in recombinant preparation of prenylated proteins and in obtaining prenylated Rab GTPases in defined nucleotide-bound states. Therefore, it was technically difficult to analyze the interaction between GDP/GTP-bound prenylated Rab and its regulators REP and GDI. 

In mammalian cells, autophagosomes initiate from isolation membranes (IMs). IMs expand, enfold cytosol, and finally close, forming autophagosomes. Microtubule-associated protein light chain 3 (LC3, the mammalian homolog 

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Adaptor Proteins. Figure 1 Adaptor protein domains. A scheme of the domain structures of some well-characterized adaptor proteins is shown. Descriptions of domain characteristics are in main text except C2, binds to phospholipids GTPase activating protein (GAP) domain, inactivates small GTPases such as Ras Hect domain, enzymatic domain of ubiquitin ligases and GUK domain, guanylate kinase domain. For clarity, not all domains contained within these proteins are shown. 

Small GTPases of the Rho family are ADP-ribosylated (e.g., at Asn4l of RhoA) and inactivated by C3-like toxins from Clostridium botulinum, Clostridium limosum, and Staphylococcus aureus. These proteins have a molecular mass of 23-30 kDa and consist only of the enzyme domain. Specific inhibition of Rho functions (Rho but not Rac or Cdc42 are targets) is the reason why C3 is widely used as a pharmacological tool [2]. 

Heterotrimeric GTP-binding Proteins Small GTPases Bacterial Toxins... 

In the cAMP-bound conformation, cAMP-GEFs specifically bind to Ras-like small GTPases and activate these proteins by profoundly accelerating the exchange of GDP for GTP. 

Guanine Nucleotide Dissociation Inhibitors. Small GTPases... 

Guanine nucleotide dissociation inhibitors (GDIs) bind to small GTPases and inhibit the dissociation and thus the exchange of the bound nucleotide. 

Guanine nucleotide exchange factors (GEFs) are proteins which catalyse the release of nucleotide bound to small GTPases. 

Small GTPases are monomeric 20 to 40 kD GTP-binding proteins that interconvert between an active (GTP-bound) and an inactive (GDP-bound) state. As molecular switches they are involved in the regulation of complex cellular processes. 

All small GTPases (except Ran) are post-translationally modified. Most important is the isoprenylation of the C-terminus. The type of modification is determined by... 

The superfamily of small GTPases consists of more than 100 members from yeast to human with more than 80 members expressed in mammalian cells. Based on structural and functional similarities the GTPases are subdivided into five major classes. 

A subfamily of Rho proteins, the Rnd family of small GTPases, are always GTP-bound and seem to be regulated by expression and localization rather than by nucleotide exchange and hydrolysis. Many Rho GTPase effectors have been identified, including protein and lipid kinases, phospholipase D and numerous adaptor proteins. One of the best characterized effector of RhoA is Rho kinase, which phosphorylates and inactivates myosin phosphatase thereby RhoA causes activation of actomyosin complexes. Rho proteins are preferred targets of bacterial protein toxins ( bacterial toxins). 

Small GTPases are not isolated molecular switches regulating cellular processes. Signalling cascades within one subfamily as well as cross-talk between... 

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