Allosteric switching by metalloregulatory proteins: Coupled equilibria and sensory networks

David Giedroc (Chemistry, Indiana University, Bloomington, USA)

Abstract:All cells respond to a changing landscape of transition metal availability by homeostatically controlling the expression of genes encoding membrane transporters (uptake and efflux systems) and metal trafficking/sequestration proteins that collectively control the intracellular availability of essential metals. This homeostatic control is mediated by a panel of specialized metal-binding proteins termed metalloregulatory or `metal sensor' proteins that function as metal-specific allosteric switches and ultimately as arbiters of metal speciation in the cell. Both thermodynamic and kinetic models of metalloregulation by metal sensor protein shave been advanced in recent years with a long-term goal to model these regulatory networks. We have employed a simple coupled equilibrium model as an artificial yet powerful construct to understand molecular determinants of allosteric regulation of DNA operator binding (ΔGc) in a number of metalloregulatory protein families, distinct from those determinants that dictate the affinity (KMe) of cognate vs. non-cognate metal (Me) binding. A key feature of these sensory networks that emerges from our studies is that the first metal coordination shell dictates metal selectivity and likely tunes KMe in an appropriate range, while the second coordination shell, organized only upon binding a cognate metal, drives biological regulation in the cell. Proper tuning of KMe may well allow weakly competitive transition metals, e.g., iron and manganese, to perform their specialized functions in the presence of highly competitive metals, e.g., copper(I) and zinc. Companion structural, thermodynamic and cell biological approaches will be discussed with a focus on zinc homeostasis in the Gram-positive respiratory pathogen, Streptococcus pneumoniae and the molecular mechanisms by which a cellular response to a excess zinc impacts manganese homeostasis.