Phosphorylation regulates KSR1 stability, ERK activation, and cell proliferation

Kinase suppressor of Ras (KSR) is a molecular scaffold that interacts with the components of the Raf/MEK/ERK kinase cascade and positively regulates ERK signaling. Phosphorylation of KSR1, particularly at Ser³⁹², is a critical regulator of KSR1 subcellular localization and ERK activation. We examined the role of phosphorylation of both Ser³⁹² and Thr ²⁷⁴ in regulating ERK activation and cell proliferation. We hypothesized that KSR1 phosphorylation is involved in generating signaling specificity through the Raf/MEK/ERK kinase cascade in response to stimulation by different growth factors. In fibroblasts, platelet-derived growth factor stimulation induces sustained ERK activation and promotes S-phase entry. Treatment with epidermal growth factor induces transient ERK activation but fails to drive cells into S phase. Mutation of Ser³⁹² and Thr ²⁷⁴ (KSR1.TVSA) promotes sustained ERK activation and cell cycle progression with either platelet-derived growth factor or epidermal growth factor treatment. KSR1^-/- mouse embryo fibroblasts expressing KSR1.TVSA proliferate two times faster and grow to a higher density than cells expressing the same level of wild-type KSR1. In addition, KSR1.TVSA is more stable than wild-type KSR1. These data demonstrate that phosphorylation and stability of the molecular scaffold KSR1 are critical regulators of growth factor-specific responses that promote cell proliferation.