Developmental patterning function of GNOM ARF-GEF mediated from the plasma membrane

Abstract

The GNOM (GN) Guanine nucleotide Exchange Factor for ARF small GTPases (ARF-GEF) is among the best studied trafficking regulators in plants, playing crucial and unique developmental roles in patterning and polarity. The current models place GN at the Golgi apparatus (GA), where it mediates secretion/recycling, and at the plasma membrane (PM) presumably contributing to clathrin-mediated endocytosis (CME). The mechanistic basis of the developmental function of GN, distinct from the other ARF-GEFs including its homologue GNOM-LIKE1 (GNL1), remains elusive. Insights from this study redefine the current notions of GN function. We show that GN, but not GNL1, localizes to the PM at long-lived structures distinct from clathrin-coated pits, while CME and secretion proceed normally in gn knockouts. The functional GN mutant variant GNfewerroots, absent from the GA, suggests that PM is the major place of GN action responsible for its developmental function. Following inhibition by Brefeldin A, GN, but not GNL1, relocates to the PM likely on exocytic vesicles, suggesting selective molecular associations. A study of GN-GNL1 chimeric ARF-GEFs indicate that all GN domains contribute to the specific GN function in a partially redundant manner. Together, this study offers significant steps towards the elucidation of the mechanism underlying unique cellular and development functions of GN.