Functional Analysis of AtVps9, a Guanine Nucleotide Exchange Factor for Rab5-related GTPases in Arabidopsis

Abstract

Arabidopsis thaliana has two different types of Rab5-related GTPases (conventional-type Ara7 and Rha1 and plant-unique-type Ara6). We have found that only one guanine nucleotide exchange factor (GEF), AtVps9a, can activate all Rab5 members in vitro. In the atvps9a-1 mutant, whose GEF activity is completely lost, embryogenesis is arrested at the torpedo stage and cells hypertrophied and aligned irregularly. GFP-Ara7 and Ara6-GFP diffused in cytosol like GDP-fixed form of Rab5s, suggesting that both types are not activated in atvps9a-1. In the leaky allele, atvps9a-2, elongation of the primary root was severely affected. Overexpression of GTP-fixed form of Ara7 suppressed the atvps9a-2 mutation, but that of Ara6 had no apparent effects. These results indicate that the two types of plant Rab5 members are functionally differentiated, even though they are regulated by the same activator, AtVps9a.