InterPro domain: IPR017231

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 [ 1 , 2 ]. 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 [ 3 ].

Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [ 4 , 4 ]. 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 Mg2 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 Mg2 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 [ 5 ].

The small GTPase superfamily can be divided into at least 8 different families, including:

• Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.
• Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.
• Rab small GTPases. GTP-binding proteins involved in vesicular traffic.
• Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.
• Ras small GTPases. GTP-binding proteins involved in signalling pathways.
• 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).
• Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.
• Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.

This entry includes Tem1 from budding yeasts and Spg1 from fission yeasts. They are GTPases involved in the regulation of the cell cycle.

In Schizosaccharomyces pombe, Spg1 is required for the localisation of Cdc7 (part of the septation initiation network) to the spindle pole body (SPB) [ 6 ]. It is regulated negatively by a GTPase-activating protein (GAP) comprising two subunits - Byr4 and Cdc16. In anaphase B, Spg1 is localised on the new SPB [ 7 ].

In Saccharomyces cerevisiae, Tem1 is associated with the mitotic exit network (MEN). It is involved in termination of M phase of the cell cycle [ 8 ].

1. The GTPase superfamily: a conserved switch for diverse cell functions. Nature 348, 125-32
2. The GTPase superfamily: conserved structure and molecular mechanism. Nature 349, 117-27
3. The ras protein family: evolutionary tree and role of conserved amino acids. Biochemistry 30, 4637-48
4. Refined crystal structure of the triphosphate conformation of H-ras p21 at 1.35 A resolution: implications for the mechanism of GTP hydrolysis. EMBO J. 9, 2351-9
5. Structure of small G proteins and their regulators. Acta Biochim. Pol. 48, 829-50
6. The Spg1p GTPase is an essential, dosage-dependent inducer of septum formation in Schizosaccharomyces pombe. Genes Dev. 11, 1519-34
7. Homoeostasis between the GTPase Spg1p and its GAP in the regulation of cytokinesis in S. pombe. J. Cell. Sci. 121, 601-8
8. Checkpoint control of mitotic exit--do budding yeast mind the GAP? J. Cell Biol. 172, 331-3