A HOMOLOG OF THE MAMMALIAN GTPASE RAB2 IS PRESENT IN ARABIDOPSIS AND IS EXPRESSED PREDOMINANTLY IN POLLEN GRAINS AND SEEDLINGS

Vesicle traffic between the endoplasmic reticulum and the Golgi appara tus in mammals requires the small GTP-binding protein Rab2, but Saccha romyces cerevisiae appears not to have a Rab2 homolog, Here it is show n that the higher plant, Arabidopsis thaliana, contains a gene, At-RAB 2, whose predicted product shares 79% identity with human Rab2 protein , Transgenic plants containing fusions between beta-glucuronidase and sequences upstream of At-RAB2 demonstrated histochemical staining pred ominantly in maturing pollen and rapidly growing organs of germinating seedlings. beta-glucuronidase activity in pollen is first detectable at microspore mitosis and increases thereafter, In this respect, the p romoter of At-RAB2 behaves like those of class II pollen-specific gene s, whose products are often required after germination for pollen tube growth. Seedling germination and pollen tube growth are notable for t heir unusually high rates of cell wall and membrane biosynthesis, Thes e results are consistent with a role for AT-RAB2 in secretory activity .