UCSF

In mammalian cells, the Ras-ERK module is deployed to couple external stimuli to cell growth, differentiation, and division. Feedback circuits impinging on this signaling module sculpt the amplitude, duration and frequency response dynamics of the terminal kinase in the cascade, ERK. In B cells, Ras activation is proposed to be digital or switch-like, and this switch-like activation is thought to propagate downstream to ERK and other effectors. Here we use live imaging of endogenous Ras and ERK activity to investigate the input-output relationship between these nodes in the context of B cell activation. We find that Ras activation is switch-like (nH = 3) and requires less receptor stimuli to maintain to initiate, consistent with hysteresis in the dose-response curve. Surprisingly, we found that a specific threshold level of Ras activity corresponded to ERK activation at the single cell level and that digital ERK responses persisted in the absence of positive feedback-mediated Ras activation. Collectively, these findings suggest that multiple sources of signal amplification exist within the Ras-ERK module of the BCR pathway.