Emergence and Enhancement of Ultrasensitivity through Posttranslational Modulation of Protein Stability

Abstract

Signal amplification converts a linear input to a steeply sigmoid output and is central to cellular functions. One canonical signal amplifying motif is zero-order ultrasensitivity through the posttranslational modification (PTM) cycle signaling proteins. The functionality of this signaling motif has been examined conventionally by supposing that the total amount of the protein substrates remains constant. However, covalent modification of signaling proteins often results in changes in their stability, which affects the abundance of the protein substrates. Here we use a mathematical model to explore the signal amplification properties in such scenarios. Our simulations indicate that PTM-induced protein stabilization brings the enzymes closer to saturation, and as a result, ultrasensitivity may emerge or is greatly enhanced, with a steeper sigmoidal response of higher magnitude and generally longer response time. In cases where PTM destabilizes the protein, ultrasensitivity can be regained through changes in the activities of the involved enzymes or from increased protein synthesis. Interestingly, ultrasensitivity is not limited to modified or unmodified protein substrates; the total protein substrate can also exhibit ultrasensitivity. It is conceivable that cells use inducible protein stabilization as a way to boost signal amplification while saving energy by keeping the protein substrate at low basal conditions.