Abstract
Thermodynamic stability represents one important constraint on protein evolution, but the molecular basis for how mutations that change stability impact fitness remains unclear. Here, we demonstrate that a prevalent global suppressor mutation in TEM β-lactamase, M182T, increases fitness by reducing proteolysis in vivo. We also show that a synthetic mutation, M182S, can act as a global suppressor and suggest that its absence from natural populations is due to genetic inaccessibility rather than fundamental differences in the protein's stability or activity.