Duplication of genes in an ATP binding cassette transport system increases dynamic range while maintaining ligand specificity [Protein Structure and Folding]
September 10th, 2014 by Ghimire-Rijal, S., Lu, X., Myles, D. A., Cuneo, M. J.
Many bacteria exist in a state of feast or famine where high nutrient availability leads to periods of growth followed by nutrient scarcity and growth stagnation. To adapt to the constantly changing nutrient flux, metabolite acquisition systems must be able to function over a broad range. This however creates difficulties as nutrient concentrations vary over many orders of magnitude, requiring metabolite acquisition systems to simultaneously balance ligand specificity and the dynamic range in which a response to a metabolite is elicited. Here we present how a gene duplication of a periplasmic binding protein (PBP) in a mannose ABC transport system potentially resolves this dilemma through gene functionalization. Determination of ligand binding affinities and specificities of the gene duplicates with fluorescence and circular dichroism demonstrate that although the binding specificity is maintained the Kd values for the same ligand differ over three orders of magnitude. These results suggest this metabolite acquisition system can transport ligand at both low and high environmental concentrations while preventing saturation with related and less preferentially metabolized compounds. The X-ray crystal structures of the beta mannose bound proteins help clarify the structural basis of gene functionalization and reveal that affinity and specificity are potentially encoded in different regions of the binding site. These studies suggest a possible functional role and adaptive advantage for the presence of two PBPs in ABC transport systems and a way bacteria can adapt to varying nutrient flux through functionalization of gene duplicates.