A phosphorylation-driven ubiquitination switch for cell-cycle control.

Cellular changes in state can be dictated by complex all-or-nothing switches built from ultrasensitive protein kinase cascades, positive-feedback loops and other mechanisms. Recent work has established that phosphorylation-driven protein destruction through the SCF ubiquitin-ligase pathway can also occur in a switch-like manner. In this context, multiple phosphorylation events are used to set a threshold for substrate targeting, thereby providing a framework for understanding the inter-relationship between protein phosphorylation and ubiquitin-mediated proteolysis.