GTPase-activating protein (GAP) for ADP ribosylation factor 1 (ARF1). May regulate coatomer-mediated protein transport from the Golgi complex to the endoplasmic reticulum. Hydrolysis of ARF1-bound GTP may lead to dissociation of coatomer from Golgi-derived membranes to allow fusion with target membranes (By similarity). Interacts with the coatomer complex. Interacts with the C-terminal appendage domain of COPG1 (By similarity). Also found on peripheral punctate structures likely to be endoplasmic reticulum-Golgi intermediate compartment. Highly expressed in liver, heart and kidney. Low expression in skeletal muscle and spleen. High levels in mammary glands of virgin mice and in mammary glands of pregnant mice associated with extensive proliferation of ductal cells and lobulo-alveolar development. Expression declines at the beginning of lactation when the glands fully differentiate. No expression after day 2 of lactation until day 21. Expression may be controlled by ID1.