<p>Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [<cite idref="PUB00052600"/>, <cite idref="PUB00004087"/>]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [<cite idref="PUB00000348"/>].</p><p>Crystallographic analysis of various small G proteins revealed the presence of a 20 kDa catalytic domain that is unique for the whole superfamily [<cite idref="PUB00004087"/>, <cite idref="PUB00023196"/>]. The domain is built of five alpha helices (A1-A5), six beta-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg(2+) and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg(2+) binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [<cite idref="PUB00015117"/>].</p><p>The small GTPase superfamily can be divided into at least 8 different families, including:</p><ul> <li>Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.</li> <li>Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.</li> <li>Rab small GTPases. GTP-binding proteins involved in vesicular traffic.</li> <li>Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.</li> <li>Ras small GTPases. GTP-binding proteins involved in signalling pathways.</li> <li>Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).</li> <li>Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.</li> <li>Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.</li> </ul><p>This entry represents a branch of the small GTPase superfamily that includes the ADP ribosylation factor Arf, Arl (Arf-like), Arp(Arf-related proteins) and the remotely related Sar (Secretion-associated and Ras-related) proteins. Arf proteins are major regulators of vesicle biogenesis in intracellular traffic [<cite idref="PUB00009830"/>]. They cycle between inactive GDP-bound and active GTP-bound forms that bind selectively to effectors. The classical structural GDP/GTP switch is characterised by conformational changes at the so-called switch 1 and switch 2 regions, which bind tightly to the gamma-phosphate of GTP but poorly or not at all to the GDP nucleotide. Structural studies of Arf1 and Arf6 have revealed that although these proteins feature the switch 1 and 2 conformational changes, they depart from other small GTP-binding proteins in that they use an additional, unique switch to propagate structural information from one side of the protein to the other. </p> <p> The GDP/GTP structural cycles of human Arf1 and Arf6 feature a unique conformational change that affects the beta2-beta3 strands connecting switch 1 and switch 2 (interswitch) and also the amphipathic helical N terminus. In GDP-boundArf1 and Arf6, the interswitch is retracted and forms a pocket to which the N-terminal helix binds, the latter serving as a molecular hasp to maintain the inactive conformation. In the GTP-bound form of these proteins, the interswitch undergoes a two-residue register shift that pulls switch 1 and switch 2 up, restoring an active conformation that can bind GTP. In this conformation, the interswitch projects out of the protein and extrudes the N-terminal hasp by occluding its binding pocket.</p>