The characterization of two proteins that are involved in the cytoskeletal architecture of the bacterium Prosthecobacter dejongeii has provided a pervasive example of potential horizontal gene transfer from a eukaryotic cell to a prokaryote.

The eukaryotic cytoskeleton contains microtubules composed of αβ-tubulin heterodimers, proteins that are thought to be unique to eukaryotes. The bacterial homologue of αβ-tubulin is the protein FtsZ, which is evolutionarily distant to αβ-tubulin, as indicated by the weak sequence and structural similarities. However, genomic analysis of P. dejongeii revealed the presence of two genes, btubA and btubB, that shared significantly higher sequence similarity with the eukaryotic αβ-tubulin than with the bacterial homologue FtsZ. Now, writing in a recent issue of Proc. Natl Acad. Sci. USA, Jan Löwe and colleagues describe the structural and biochemical characterization of BtubA and BtubB. Biochemical investigation of the proteins indicated that some of their properties, including protein folding and dimerization, resembled those of FtsZ rather than αβ-tubulin; chaperones are not required for proper folding and BtubA and BtubB do not form a tight heterodimer. However, when the authors solved the crystal structures of BtubA and BtubB, they were shown to be strikingly similar to the structure of eukaryotic tubulin. Indeed, the structure of BtubA/B is much closer to αβ-tubulin than to FtsZ. Furthermore, the BtubA/B structure contains elements such as surface loops and specific domains that are only necessary in a eukaryotic context.

Taken together, this analysis suggests that the most plausible explanation for the origin of btubA and btubB — the only genes in the P. dejongeii genome with a high degree of similarity to eukaryotic genes — is that tubulin gene(s) were horizontally transferred to Prosthecobacter from a eukaryotic cell.