The Caulobacter crescentus CgtAC Protein Cosediments with the Free 50S Ribosomal Subunit

Abstract

The Obg family of GTPases is widely conserved and predicted to play an as-yet-unknown role in translation. Recent reports provide circumstantial evidence that both eukaryotic and prokaryotic Obg proteins are associated with the large ribosomal subunit. Here we provide direct evidence that the Caulobacter crescentus CgtAC protein is associated with the free large (50S) ribosomal subunit but not with 70S monosomes or with translating ribosomes. In contrast to the Bacillus subtilis and Escherichia coli proteins, CtAC does not fractionate in a large complex by gel filtration, indicating a moderately weak association with the 50S subunit. Moreover, binding of CgtAC to the 50S particle is sensitive to salt concentration and buffer composition but not guanine nucleotide occupancy of CgtAC. Assays of epitope-tagged wild-type and mutant variants of CgtAC indicate that the C terminus of CgtAC is critical for 50S association. Interestingly, the addition of a C-terminal epitope tag also affected the ability of various CgtAC alleles to function in vivo. Depletion of CgtAC led to perturbations in the polysome profile, raising the possibility that CgtAC is involved in ribosome assembly or stability.