Molecular stripping in the NF-κB/IκB/DNA genetic regulatory network

Abstract

Genetic switches based on the NF-κB/ IκB/ DNA system are master regulators of an array of cellular responses. Recent kinetic experiments have shown that IκB can actively remove NF-κB bound to its genetic sites via a process called "molecular stripping." This allows the NF-κB/ IκB/ DNA switch to function under kinetic control rather than the thermodynamic control contemplated in the traditional models of gene switches. Using molecular dynamics simulations of coarse-grained predictive energy landscape models for the constituent proteins by themselves and interacting with the DNA we explore the functional motions of the transcription factor NF-κB and its various binary and ternary complexes with DNA and the inhibitor IκB. These studies show that the function of the NF-κB/ IκB/ DNA genetic switch is realized via an allosteric mechanism. Molecular stripping occurs through the activation of a domain twist mode by the binding of IκB that occurs through conformational selection. Free energy calculations for DNA binding show that the binding of IκB not only results in a significant decrease of the affinity of the transcription factor for the DNA but also kinetically speeds DNA release. Projections of the free energy onto various reaction coordinates reveal the structural details of the stripping pathways. T he binding and release of protein transcription factors from DNA are fundamental molecular processes by which genes are regulated in the cell. The pioneering studies of Jacob, Monod, Ptashne, and Gilbert explained how these two processes, seeming inverses of each other, while being maintained in local chemical equilibrium, could still lead to robust genetic switches by coupling to protein synthesis and degradation, which are kinetically controlled far from equilibrium processes (1-4). This classic picture, with the law of mass action at its core (5, 6), suggests that understanding the molecular mechanism of the binding and release of transcription factors is of secondary interest compared with understanding the thermodynamics of protein-DNA recognition. The recent discovery of proteininduced release of a DNA-bound transcription factor in the NF-κB=IκB=DNA genetic switch changes this picture (7). The induced process, called "molecular stripping," opens up the possibility of molecular kinetic control of binding and release, thus overturning the classic paradigm based only on thermodynamic control. In this paper, we use molecular dynamics simulations of coarse-grained but predictive energy landscape models of the proteins along with their interacting DNA to explore first how the NF-κB transcription factor binds individually both to DNA and to its inhibitor IκB and then to study how an approaching IκB can strip the NF-κB from a DNA molecule to which it has already been bound, by forming an intermediate ternary complex. These simulations show that each of the binary binding events involves conformational selection of different NF-κB global conformations. Molecular stripping then occurs that is induced by forming the ternar...

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