Mechanism of action of the flippase Drs2p in modulating GTP hydrolysis of Arl1p.

Small GTPase ADP-ribosylation factors (ARFs) are key regulators of membrane trafficking and their activities are determined by guanine-nucleotide-binding status. In Saccharomyces cerevisiae, Arl1p, an ARF-like protein, is responsible for multiple trafficking pathways at the Golgi. The GTP-hydrolysis activity of Arl1p is stimulated by its GTPase-activating protein Gcs1p, and binding with its effector Imh1p protects Arl1p from premature inactivation. However, the mechanism involved in the timing of Arl1p inactivation is unclear. Here, we demonstrate that another Arl1p effector, the lipid flippase Drs2p, is required for Gcs1p-stimulated inactivation of Arl1p. Drs2p is known to be activated by Arl1p and is involved in vesicle formation through its ability to create membrane asymmetry. We found that the flippase activity of Drs2p is required for proper membrane targeting of Gcs1p in vivo. Through modification of the membrane environment, Drs2p promotes the affinity of Gcs1p for the Golgi, where it binds to active Arl1p. Together, Imh1p and Drs2p modulate the activity of Gcs1p by timing its interaction with Arl1p, hence providing feedback regulation of Arl1p activity.