An important goal of synthetic biology is to construct reaction circuits with artificial responses by assembling modulated biological elements into living cells. While many such attempts have been based upon the cellular transcriptional apparatus, the use of the post-translational machinery remains relatively rare. Here we report the reconstruction in Escherichia coil of a protein-based artificial module based upon elements of a eukaryotic cell signaling pathway. The module shows a switch-like ultrasensitive response, using the opposing functions of a protein kinase and a phosphatase. The switch is acutely responsive to the kinase:phosphatase ratio, and can be modulated as a function of the expression level of the substrate. We can theoretically predict the response of this module and can control its steepness based on these predictions. Future work will demonstrate the potential of this controllable protein-based switch to be incorporated into artificial circuits. (C) 2012 Elsevier Inc. All rights reserved.