Vesicle traffic between the endoplasmic reticulum and the Golgi apparatus in mammals requires the small GTP-binding protein Rab2, but Saccharomyces cerevisiae appears not to have a Rab2 homolog. Here it is shown that the higher plant, Arabidopsis thaliana, contains a gene, At-RAB2, 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 predominantly 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 promoter of At-RAB2 behaves like those of class II pollen-specific genes, whose products are often required after germination for pollen tube growth. Seedling germination and pollen tube growth are notable for their unusually high rates of cell wall and membrane biosynthesis. These results are consistent with a role for At-RAB2 in secretory activity.

Biologia plantarum, an international journal for experimental botany founded in 1959 by Professor Bohumil Němec. Covers all branches of experimental botany ranging from molecular biology and biotechnology to whole-plant and stand functioning.

Small G-proteins are encoded by ras-related genes and play a central role in cell differentiation and vesicle transport. Here we describe experiments to identify members of this gene family in higher plants. Several oligonucleotide mixtures, corresponding to strictly conserved amino acid sequences in all the ras and ras-related proteins, from yeast to man, were used to search for members of the plant ras gene family. With these probes we initially screened a coleoptile-specific cDNA library from Zea mays (L.) and isolated two genes, denoted yptm1 and yptm2, encoding proteins homologous to the ypt/rab protein family. Using these cDNAs as probes additional members of the ypt/rab gene family were isolated from Zea mays and Arabidopsis thaliana. The proteins predicted from the cDNAs isolated from maize and Arabidopsis showed high similarity to other members of the ras family, in particular in the regions involved in GTP/GDP binding, GTPase activity, and membrane binding. Homology to the ypt/rab family was in the range 70-80%, whereas similarity to p21ras and other ras-related genes was less than 40%. In addition, we identified genes from maize and Arabidopsis, yptm3 and At-rab2, encoding proteins homologous to the mammalian rab2 protein. These genes were expressed preferentially in male flower tissue, in particular in the anthers and pollen. This expression pattern was confirmed by expression of the E. coli beta-glucuronidase gene under the control of the At-rab2 promoter. We observed that, in A. thaliana plants stably transformed with this construct, the reporter gene was predominantly expressed in vascular tissues and in pollen.

We have isolated, cloned, and characterized two cDNAs from Zea mays (L.), denoted yptm1 and yptm2, encoding proteins related to the ypt protein family. Amino acid similarity scores with YPT1 from yeast and ypt from mouse are in the range of 70% for yptm1 and 74% for yptm2, respectively, whereas similarities with p21 ras and other ras-related proteins are less than 40%. Most amino acid residues showing identity are clustered in the GTP/GDP binding domain. In addition, two cysteine residues close to the C-terminal ends, known to be palmitoylated and necessary for membrane binding in all eukaryotic ras-related proteins that have been characterized so far, are conserved in the maize genes as well. Northern blot hybridization analysis of poly(A)+ mRNA from etiolated maize coleoptiles revealed single mRNA species of approximately the same size as the isolated cDNAs. The gene for yptm1 is expressed at very low levels in maize coleoptiles and tissue culture cells. The gene for yptm2 is expressed at higher levels and is differentially represented in RNAs isolated from various organs of maize plants, with its highest level in leaves and flowers. The structural similarity of the genes identified suggests that they could be involved in the control of secretory processes.