The GTP binding protein Obg homolog ObgE is involved in ribosome maturation

Abstract

Obg proteins belong to a subfamily of GTP binding proteins, which are highly conserved from bacteria to human. Mutations of obgE genes cause pleiotropic defects in various species but the function remained unclear. Here we examine the function of ObgE, the Obg homolog in Escherichia coli . The growth rate correlates with the amount of ObgE in cells. Co-fractionation experiments further suggest that ObgE binds to 30S and 50S ribosomal subunits, but not to 70S ribosome. Pull-down assays suggest that ObgE associates with several specific ribosomal proteins of 30S and 50S subunits, as well as RNA helicase CsdA. Purified ObgE cosediments with 16S and 23S ribosomal RNAs in vitro in the presence of GTP. Finally, mutation of ObgE affects pre-16Sr-RNA processing, ribosomal protein levels, and ribosomal protein modification, thereby significantly reducing 70S ribosome levels. This evidence implicates that ObgE functions in ribosomal biogenesis, presumably through the binding to rRNAs and/or rRNA-ribosomal protein complexes, perhaps as an rRNA/ribosomal protein folding chaperone or scaffold protein.

Chromosome segregation control by Escherichia coli ObgE GTPase

“…Both E. coli and C. crescentus have been reported to copurify with the ribosome, although this interaction has been reported to be weak (for Caulobacter) (Lin et al, 2004) or variable (for E. coli ) (Kobayashi et al, 2001;Sato et al, 2005). It has been suggested that ObgE is required for ribosome biogenesis and that observed cell cycle effects may be an indirect consequence of loss of translation capacity (Sato et al, 2005).…”

“…Previous studies of Obg orthologues have examined different temperature-sensitive alleles and point mutations to deplete functional Obg in Gram-negative bacteria (Kobayashi et al, 2001;Datta et al, 2004;Sato et al, 2005;Sikora et al, 2006). The results of these studies have suggested various and diverse cellular roles, including ribosome biogenesis, stress response and chromosome segregation.…”

“…Obg from Streptomyces ceolicolor controls mycelium formation in response to intracellular pools of guanine nucleotides (Okamoto and Ochi, 1998). A mutational analysis of the C. crescentus orthologue suggests accumulation of 50S ribosomal subunits at the expense of 70S assembled ribosomes (Lin et al, 2004), a phenotype also seen in E. coli (Sato et al, 2005). However, the essential function of Caulobacter Obg (CgtA) appears to be progression through the cell cycle (Datta et al, 2004) rather than defects in ribosome assembly.…”

Study on the chaperone properties of conserved GTPases

“…Both proteins have a binding site on ribosomes, and they could interact with L23, a ribosomal protein on the peptide exit tunnel of ribosomes (Gu et al, 2003). Era, ObgE, SelB, and YchF could also interact with ribosomes (Sayed et al, 1999;Leipe et al, 2002;Sato et al, 2005). The binding of these GTPases to ribosomes reduced the amount of dissociated proteins, which could improve denatured citrate synthase refolding, thereby decreasing the reactivation level.…”

Trypanosoma brucei Mitochondrial Ribosomes

“…The other motifs indicate GTP binding, GTPase, methyltransferase, peptidyl-prolyl isomerase, helicase activities, and chaperone function. These proteins may have roles in ribosome maturation or assembly, RNA folding, rRNA processing, protein assembly, and subunit structure stabilization similar to what has been observed in other organisms (41)(42)(43)(44)(45)(46)(47)(48)(49). Intriguingly helicase activity was recently identified in ribosomes that could account for its ability to translate through downstream mRNA secondary structure (50).…”