Toward a model for Arf GTPases as regulators of traffic at the Golgi

Abstract

In this review, I summarize the likely roles played by ADP-ribosylation factor (Arf) proteins in the regulation of membrane traffic at the Golgi, from the perspective of the GTPase. The most glaring limitations to the development of a coherent molecular model are highlighted; including incomplete information on the initiation of Arf activation, identification of the "accessory proteins" required for carrier maturation and scission, and those required for directed traffic and fusion at the destination membrane. Though incomplete, the molecular model of carrier biogenesis has developed rapidly in recent years and promises richness in understanding this essential process.

Fig. 1. Summary of Arf interactions at the Golgi

Arfs are present in cytosol of all eukaryotic cells and interact with (predominantly planar) membranes, where the actions of Arf GEFs (GBF1, Big1, and Big2 are implicated at Golgi) promote binding of GTP and activation of Arf that stabilizes binding to the bilayer. The Arf-GTP assumes the activated conformation, which has increased affinity for multiple effectors that include protein adaptors for coupling to cargos, lipid modifying enzymes for generating local changes in membrane and additional binding sites for other proteins, and Arf GAPs (e.g., ArfGAP1–3) that may act as effectors to recruit other components to a nascent bud, or promote GTP hydrolysis by Arf and return to its less active, GDP-bound conformation. The activation of Arf on the membrane also is accompanied by insertion of the myristate into the bilayer, binding of the amphipathic α-helix with surface head groups and, along with effector recruitment and localized changes in lipids, promotes membrane curvature and budding of nascent carriers.

Fig. 2. Building a model for Arf regulation of carrier biogenesis at the Golgi

Almost every single step shown has been documented, but the specific contributions of most steps to carrier biogenesis at the Golgi remain somewhat uncertain. Most speculative is step 1, in which the accumulation of cargo is shown as the initiating event in coat protein assembly, through recruitment of an Arf GEF. The cargo is shown as a transmembrane protein with cytoplasmic tail that contains sorting motifs, such as those shown on the left most cargo. Soluble Arfs are proposed to “sample” planar membranes (step 2) independently of other events, but when an Arf GEF is encountered, the Arf is activated (step 3) and becomes more stably bound to the membrane as a result of insertion of the myristate into the bilayer and binding of the N-terminal helix to lipid head groups. The different steps 4 represent Arf recruitment (4a–d) and activation (4d–e) of various effectors that have each been implicated in aspects of carrier biogenesis at the Golgi. The deformation of the planar membrane is not shown, but the mature severed carrier is shown at the right (step 5). Note the absence of Arf in the mature carrier and predicted presence of cargo, adaptor, Arf GAP, and lipid products. The components depicted are not to scale, but rather each is shown as similar in size, to highlight its likely role in Arf actions at the Golgi.