Schematic description of adaptation pathways and their properties.

<p>(a) Bacterial chemotaxis. Attractant molecules bind to chemoreceptors, which cluster due to adapter protein CheW and kinase CheA, responsible for phosphorylation of CheY and hence regulation of the flagellar motors. CheR and CheB (activated by CheA) mediate adaptation by methylating and demethylating receptors, respectively <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Kentner1" target="_blank">[43]</a>. (b) <i>Dictyostelium</i> chemotaxis, as related to the production and sensing of cAMP. After the binding of cAMP to the receptor, the G-protein complex dissociates <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Manahan1" target="_blank">[6]</a> and RasG protein is activated <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Takeda1" target="_blank">[15]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Iglesias1" target="_blank">[44]</a>. The adenylyl cyclase (AC), possibly through the pathway <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Manahan1" target="_blank">[6]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Kimmel1" target="_blank">[45]</a>, is activated, and produces cAMP, secreted for cell aggregation. The concentration of cAMP is also increased through ERK2, which inhibits the phosphodiesterase (PDE) RegA, in turn hydrolyzing cAMP. This pathway is inhibited by PKA, activated by cAMP <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-McMains1" target="_blank">[19]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Kimmel1" target="_blank">[45]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Valeyev1" target="_blank">[46]</a>. (c) Photo-transduction. Light activates rhodopsin () and, following a G-protein cascade, phosphodiesterase hydrolyzes cGMP. At low PDE levels (in the dark), cGMP allows the influx of through cyclic nucleotide-gated channels (CNGC). Adaptation is mediated via -dependent feedback loops: inhibition of GC, phosphorylation of , and CNGC <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Yau1" target="_blank">[9]</a>. (d) Olfactory-transduction. Odorant binds to the G-protein coupled olfactory receptor, activating AC to produce cAMP, causing the opening of CNGC. The influx of opens chloride channels to amplify signal. Several -dependent feedback mechanisms mediate adaptation: inhibition of AC, CNGC and cAMP <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Kaupp1" target="_blank">[7]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Kleene1" target="_blank">[8]</a>. (central panel) Response of an adaptive system to a step stimulus, and characteristic features considered here, with and <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003300#pcbi.1003300-Ma1" target="_blank">[47]</a>.