Cloning and characterization of Xenopus laevis drg2, a member of the developmentally regulated GTP-binding protein subfamily.

The developmentally regulated GTP-binding protein (DRG) subfamily is an uncharacterized member of the Obg family, an evolutional branch of GTPase superfamily proteins. GTPases act as molecular switches regulating diverse cellular processes. DRG2 and DRG1 comprise the DRG subfamily in eucaryotes. Although drg1 was first identified as a gene predominantly expressed during early development of the mouse central nervous system, comparative analysis of drg2 and drg1 expression during embryogenesis has never been reported, and the biochemical properties of the DRG family proteins remain to be elucidated. Thus, we first cloned Xenopus drg2 (Xdrg2) and examined the temporal and spatial expression patterns of Xdrg2 mRNA in comparison to those of Xdrg1. Both Xdrg2 and Xdrg1 are induced at late gastrula and subsequently increased during later stages of embryos (stage 13-41). Whole-mount in situ hybridization showed that Xdrg2 and Xdrg1 expression patterns are almost identical except that only Xdrg2 expression is detected in the stage 22 pronephric anlage. Strong transcripts of both genes are also observed at this stage in neural crest cells, blood islands, and developing eyes, and in brain, eyes, otic vesicle, branchial arches, pronephroses, spinal cord, notochord, head mesenchyme, and somites at stages 27 and 32. Northern blot analysis of adult tissues revealed that both genes are expressed highly in ovary and testis and rather moderately in other organs, except that Xdrg1 transcripts are scarcely detected in heart, lung, and liver. Accordingly, transcription or stability of Xdrg2 and Xdrg1 mRNAs may be regulated by different mechanisms. In addition, by generating recombinant XDRG2 and XDRG1 proteins, we found the RNA binding activity of these proteins in vitro. Our results suggest that the DRG proteins may play their physiological roles via RNA binding.