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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]

Coatomer is a multisubunit protein complex that forms the COPI coat on transport carriers. With an ARF GTPase, it helps select cargo and deform membranes in traffic associated with the Golgi apparatus, including retrieval toward the endoplasmic reticulum. Coat assembly helps generate a carrier; later targeting and membrane fusion require additional machinery. Retrieval prevents forward traffic from progressively draining resident components from earlier compartments. ^[ref]

COPI is a coat system involved in retrieval toward the endoplasmic reticulum and traffic associated with the Golgi apparatus. Its coatomer assembly and ARF-family recruitment differ from COPII, while sharing membrane recruitment and cargo-selection principles.