Abstract
GTP-binding proteins play important roles as molecular switch in various cellular processes such as signal transduction, protein synthesis and translocation or cell cycle regulation. Obg is a member of the Obg/GTP1 subfamily that is a distinct group of monomeric GTP-binding proteins. The Obg homologues have been identified in diverse organisms ranging from bacteria to eukaryotes including human and higher plants. Bacterial Obg is typically essential for cell viability. However, their precise functions are still poorly definced. Arabidopsis contains an Obg homologue gene named AtOBG. The AtOBG showes homology to Bacillus Obg except for the N-terminal extension. The AtOBG mRNAs were accumulated preferentially in leaves in a light-dependent mannner. The AtOBG targets to chloroplasts and is likely associated with the nucleoids-like structures in chloroplasts. The AtOBG knockout embryos did not develop beyond the globular stage. This is the first experimental evidence showing that Obg is an essential organellar protein in eukaryotes.
