Feedback regulation of opposing enzymes generates robust, all-or-none bistable responses

Summary Positive feedback loops and double-negative feedback loops can generate bistability, allowing signaling networks to convert continuously graded inputs into discrete outputs [1–4]. One particularly well-studied bistable system consists of the mitotic regulator CDK1 with its inactivator Wee1 and its activator Cdc25 [5–7]. The system functions as a mitotic trigger, toggling between a stable interphase state, where CDK1 is off, and a stable M-phase state, where CDK1 is on. One striking aspect of the CDK1–Cdc25–Wee1 system is the symmetry of its two feedback loops (Figure 1). CDK1 phosphorylates Cdc25 at multiple sites in the protein's amino-terminal regulatory region, contributing to Cdc25 activation; CDK1 phosphorylates Wee1 at multiple sites in its amino terminus, inactivating the protein. Active Wee1 phosphorylates CDK1 at Tyr15, and thereby inactivates it; active Cdc25 dephosphorylates the same site, reversing the inactivation. In principle either loop alone could generate the bistable response observed in the CDK1–Cdc25–Wee1 system [6–8], yet, throughout evolution, both loops are invariably present. This basic design — reciprocal feedback regulation of opposing enzymes — can be seen in other regulatory switches as well [9,10]. Previous work has shown that when interlinked loops operate on different timescales, it can allow the system to quickly respond and then slowly lock into a noise-resistant state [9]. However, in the case of the CDK1–Cdc25–Wee1 system, the timescales of the two loops are indistinguishable. Here we show that a mirror-image, two-loop system offers an important advantage over a one-loop system even when the timescales of the two loops are identical: the symmetrical set-up makes it substantially easier to generate a bistable response.