<p>Small GTPases form an independent superfamily within the larger class ofregulatory GTP hydrolases. This superfamily contains proteins that control avast number of important processes and possess a common, structurallypreserved GTP-binding domain [<cite idref="PUB00052600"/>], [<cite idref="PUB00004087"/>]. Sequence comparisons of small G proteinsfrom various species have revealed that they are conserved in primarystructures 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), sixbeta-strands (B1-B6) and five polypeptide loops (G1-G5). A structuralcomparison of the GTP- and GDP-bound form, allows one to distinguish twofunctional loop regions: switch I and switch II that surround thegamma-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 ofthe phosphate groups. The G3 loop provides residues for Mg(2+) and phosphatebinding and is located at the N terminus of the A2 helix. The G1 and G3 loopsare sequentially similar to Walker A and Walker B boxes that are found inother nucleotide binding motifs. The G2 loop connects the A1 helix and the B2strand and contains a conserved Thr residue responsible for Mg(2+) binding.The guanine base is recognised by the G4 and G5 loops. The consensus sequenceNKXD of the G4 loop contains Lys and Asp residues directly interacting withthe nucleotide. Part of the G5 loop located between B6 and A5 acts as arecognition site for the guanine base [<cite idref="PUB00015117"/>].</p><p>The small GTPase superfamily can be divided in 8 different families:<ul><li>Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and un-coating 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 signaling 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><p> This entry represents the Roc small GTPase domain. </p> ROC GTPase