THE BIOLOGICAL ROLES OF SMALL GTPASES AND INTERACTING PROTEINS IN PLANTS

Extensive studies on the molecular mechanisms of vesicular trafficking have revealed that molecules involved in this cellular function are r emarkably well conserved from yeast to higher plants. However, it is n ot clear at all how a variety of organisms maintain the individual div ergent systems using the common machinery of vesicular traffic. We hav e been attempting to understand the roles and regulatory mechanisms of vesicular traffic in plants through the study of Rab/Ypt GTPases. Ara proteins are Rab/Ypt homologues of Arabidopsis, which are implicated in the regulation of vesicular traffic. Their biochemical properties a re similar to those of the Rab/Ypt proteins from animal and yeast cell s. The overexpression of ARA2 or ARA4 causes pleiotropic morphological abnormalities in the transgenic tobacco plants. The GTPase cycle of A ra proteins has to be strictly controlled for their proper functions. We have identified two classes of regulator molecules of Ara2 and Ara4 . One is the GTPase activating protein (GAP), and the other is the GDP dissociation inhibitor (GDI). GAP has been identified as an activity accelerating the hydrolysis of GTP by Ara2 or Ara4. GDI (AtGDI1) has b een isolated as a molecule interacting with Ara4 using a novel method for detecting interactions between foreign molecules in yeast. Further studies on the interacting molecules should unveil the regulatory sys tem of and signal transduction pathway via Ara proteins.