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. Author manuscript; available in PMC: 2006 Dec 4.
Published in final edited form as: Nat Rev Mol Cell Biol. 2006 Mar;7(3):165–176. doi: 10.1038/nrm1838

FIG. 1.

FIG. 1

Universal motifs of cellular signalling networks. A. One-site phosphorylation cycle. The protein M is phosphorylated by a kinase to yield the phosphorylated form Mp, which is dephosphorylated by an opposing phosphatase. B. A cycle of a small GTPase (Ran). A guanine nucleotide exchange factor (GEF) catalyzes the transformation of an inactive guanosine diphosphate (GDP)-bound form (Ran-GDP) into an active guanosine triphosphate (GTP)-bound form (Ran-GTP). A GTPase–activating protein (GAP) is the opposing enzyme that catalyzes the reverse transformation. C. A cascade of cycles. Negative feedback provides robustness to noise, increasing resistance to disturbances inside the feedback loop, but brings about oscillations if it is too strong and the cascade is ultrasensitive12,20. Positive feedback greatly increases the sensitivity of the target to the signal and may also lead to bistability and relaxation oscillations12,18,46,72.