Search

The specific arrangement of secondary elements in a local motif often totally relies on the formation of coordination bonds between metal ions and protein ligands. This is typified by the -30 amino acid eukaryotic zinc finger motif in which a beta-sheet and an or-helix are clustered around a zinc ion by various combinations of four ligands. The prokaryotic zinc finger domain (found in the Ros protein from Agrobacterium tumefaciens) is different from the eukaryotic counterpart as it consists of 58 amino acids arranged in a beta beta beta alpha alpha topology stabilized by a 15-residue hydrophobic core. Also, this domain tetrahedrally coordinates zinc and unfolds in the absence of the metal ion.The characterization of proteins belonging to the Ros homologs family has however shown that the prokaryotic zinc finger domain can overcome the metal requirement to achieve the same fold and DNA-binding activity. In the present work, two zinc-lacking Ros homologs (MI4 and MI5 proteins) have been thoroughly characterized using bioinformatics, biochemical and NMR techniques.We show how in these proteins a network of hydrogen bonds and hydrophobic interactions surrogate the zinc coordination role in the achievement of the same functional fold.(c) 2013 Published by Elsevier B.V.

International audience; The exact evolutionary origin of the zinc finger (ZF) domain is unknown, as it is still not clear from which organisms it was first derived. However, the unique features of the ZF domains have made it very easy for evolution to tinker with them in a number of different manners, including their combination, variation of their number by unequal crossing-over or tandem duplication and tuning of their affinity for specific DNA sequence motifs through point substitutions. Classical Cys 2 His 2 ZF domains as structurally autonomous motifs arranged in multiple copies are known only in eukaryotes. Nonetheless, a single prokaryotic Cys 2 His 2 ZF domain has been identified in the transcriptional regulator Ros from Agrobacterium tumefaciens and recently characterized. The present work focuses on the evolution of the classical ZF domains with the goal of trying to determine whether eukaryotic ZFs have evolved from the prokaryotic Ros-like proteins. Our results, based on computational and experimental data, indicate that a single insertion of three amino acids in the short loop that separates the b-sheet from the a-helix of the Ros protein is sufficient to induce a structural transition from a Ros like to an eukaryotic-ZF like structure. This observation provides evidence for a structurally plausible and parsimonious scenario of fold evolution, giving a structural basis to the hypothesis of a horizontal gene transfer (HGT) from bacteria to eukaryotes.