TI - Differential Affinity and Catalytic Activity of CheZ in E.coli Chemotaxis . Differential Affinity and Catalytic Activity . AB - Push-pull networks , in which two antagonistic enzymes control the activity of a messenger protein , are ubiquitous in signal transduction pathways . A classical example is the chemotaxis system of the bacterium Escherichia coli , in which the kinase CheA and the phosphatase CheZ regulate the PHOSphorylation level of the messenger protein CheY . Recent experiments suggest that both the kinase and the phosphatase are localized at the receptor cluster , and Vaknin and Berg recently demonstrated that the spatial distribution of the phosphatase can markedly affect the dose-response curves . We argue , using mathematical modeling , that the canonical model of the chemotaxis network cannot explain the experimental observations of Vaknin and Berg . We present a new model , in which a small fraction of the phosphatase is localized at the receptor cluster , while the remainder freely diffuses in the cytoplasm ; moreover , the phosphatase at the cluster has a higher binding affinity for the messenger protein and a higher catalytic activity than the phosphatase in the cytoplasm . This model is consistent with a large body of experimental data and can explain many of the experimental observations of Vaknin and Berg . More generally , the combination of differential affinity and catalytic activity provides a generic mechanism for amplifying signals that could be exploited in other two-component signaling systems . If this model is correct , then a number of recent modeling studies , which aim to explain the chemotactic gain in terms of the activity of the receptor cluster , should be reconsidered .