Small GTPase

A small GTPase is a relatively small nucleotide-switch protein that cycles between GDP- and GTP-bound states. Ras family proteins illustrate signaling mechanisms distinct from the three-subunit organization of a heterotrimeric G protein.

Ran is a small GTPase involved in transport through the nuclear pore complex and in organization of microtubules. It switches between GDP- and GTP-bound states. The spatial organization of its regulators makes its nucleotide state informative about location, rather than simply reporting the total amount of the protein. ^[ref]

In nuclear import, a high nuclear Ran-GTP activity promotes release of cargo from transport receptors, while the cytosolic side supports recycling through a different nucleotide state. This directional cycle depends on separated regulator activities and a supply of chemical energy. Equalizing all conditions on both sides would remove the same spatial cue even if the total Ran abundance stayed fixed. ^[ref]

A small GTPase participating in protrusion and polarity in migrating cells.

Nucleotide exchange changes recruitment of effectors that regulate the actin cytoskeleton. Spatially restricted activity can help select a protrusive region, connecting a biochemical switch to directional cell movement. ^[ref]

A Rab GTPase is a small GTP-binding protein that regulates membrane traffic. GDP- and GTP-bound states differ in their interactions with effectors, while exchange factors and GTPase-activating proteins regulate the cycle. Different Rabs act at different trafficking stages, helping organize compartment identity, motor recruitment and carrier targeting. Rab8a and Rab11a illustrate related switches with different preferred routes and partners. ^[ref]

Rab8a is a Rab GTPase involved in delivery toward the cell surface and ciliary membrane organization. Rabin8 activates it through nucleotide exchange, linking upstream signals to effector recruitment. Its membrane location and nucleotide state together help determine productive interactions. A fluorescent Rab8a spot therefore reports localization without automatically specifying whether the switch is active or transport is proceeding. ^[ref]

Rab11a is a Rab GTPase associated with recycling membrane traffic. It helps organize the return or redistribution of internalized components and contributes to polarized delivery in several cell types. Its effectors connect nucleotide state to targeting and motor systems. A recycled receptor can regain surface availability without being synthesized again, illustrating how trafficking changes cellular responsiveness on short timescales. ^[ref]

A Rab-family regulator associated with late endosomal and degradative trafficking.

Its effectors help connect late endosomes to movement, targeting and fusion with degradative compartments. The transition from Rab5 to Rab7 illustrates maturation of membrane identity; a static marker image captures a state rather than the whole trafficking history. ^[ref]

A Rab-family regulator associated with early endosomal membrane domains.

Active Rab5 recruits partners that organize early endosomes and membrane fusion. Replacement of Rab5-associated identity by Rab7-associated identity helps connect early uptake with later degradative trafficking, rather than requiring each endosome to retain one permanent molecular label. ^[ref]

Replacement of an early Rab-regulated membrane identity by a later one, exemplified by Rab5 loss and Rab7 acquisition during endosome maturation.

An ARF GTPase is a small GTP-binding protein involved in membrane organization and coat recruitment. Nucleotide exchange promotes a membrane-associated active state that recruits selected effectors, while hydrolysis contributes to turnover of the assembly. ARF1 is a prominent regulator of coatomer recruitment at Golgi-associated membranes. Different family members and effectors act in different locations, so the family name does not specify one universal cargo route. ^[ref]

Sar1 GTPase is a small GTP-binding protein that initiates COPII coat assembly at the endoplasmic reticulum. Its GTP-bound state associates with the membrane and recruits the Sec23-Sec24 complex, followed by the Sec13-Sec31 complex. Nucleotide cycling helps coordinate coat assembly and turnover. Reconstitution with defined components demonstrates a direct role in carrier formation rather than only an indirect consequence of cellular signalling. ^[ref]