Arf GTPases are key regulators of both retrograde and anterograde traffic at the Golgi complex. The Golgi-localized Arf activators, Arf-GEFs (guanine exchange factor) of the BIG/GBF family, are poorly understood in terms of both their regulatory and localization mechanisms. We have performed a detailed kinetic characterization of a functional Golgi Arf-GEF, the . trans-Golgi network (TGN)-localized Sec7 protein from yeast. We demonstrate that Sec7 is regulated by both autoinhibition and positive feedback. We show that positive feedback arises through the stable recruitment of Sec7 to membranes via its HDS1 domain by interaction with its product, activated Arf1. This interaction mediates localization of Sec7 to the TGN, because deletion of the HDS1 domain or mutation of the HDS1 domain in combination with deletion of Arf1 significantly increases cytoplasmic localization of Sec7. Our results lead us to propose a model in which Arf-GEF recruitment is linked to Golgi maturation via Arf1 activation. Arf guanine exchange factors (GEFs) activate Arf GTPases, key regulators of trafficking at the Golgi complex. Richardson et al. show that the activity and localization of the Arf-GEF Sec7 is controlled through autoinhibition and positive feedback. The authors propose that active Arf1-mediated positive feedback links Arf-GEF recruitment to Golgi maturation.
Bibliographical noteFunding Information:
We acknowledge the generosity of the Emr, Bretscher, and Schekman laboratories in sharing reagents, strains, plasmids, equipment, and advice. We are grateful to Y. Mao for assistance with insect cell culture. We thank J. MacGurn for making strains CFY403 and CFY589 and M. Rogals for making plasmid pMR1. We thank Scott Emr and Tony Bretscher for many fruitful discussions. We received helpful advice on microscopy from T. Bretscher, K. Donovan, D. Garbett, B. Judson, A. Manford, and C. Stefan. The authors are supported by Cornell University startup funding and by NIH/NIGMS grant R01GM098621.