Signaling through RAS/MAP kinase pathway is central to biology. ERK has long been perceived as the only substrate for MEK. Here, we report that HSF1, the master regulator of the proteotoxic stress response, is a new MEK substrate. Beyond mediating cell-environment interactions, the MEK-HSF1 regulation impacts malignancy. In tumor cells, MEK blockade inactivates HSF1 and thereby provokes proteomic chaos, presented as protein destabilization, aggregation, and, strikingly, amyloidogenesis. Unlike their non-transformed counterparts, tumor cells are particularly susceptible to proteomic perturbation and amyloid induction. Amyloidogenesis is tumor suppressive, reducing in vivo melanoma growth and contributing to the potent anti-neoplastic effects of proteotoxic stressors. Our findings unveil a key biological function of the oncogenic RAS-MEK signaling in guarding proteostasis and suppressing amyloidogenesis. Thus, proteomic instability is an intrinsic feature of malignant state, and disrupting the fragile tumor proteostasis to promote amyloidogenesis may be a feasible therapeutic strategy. Display omitted •MEK physically interacts with and phosphorylates HSF1 at Ser326•MEK mobilizes and ERK suppresses the HSF1-mediated proteotoxic stress response•MEK inhibition disrupts proteostasis and provokes tumor-suppressive amyloidogenesis•Malignancy is susceptible to proteomic perturbation and subsequent amyloidogenesis The identification of HSF1 as a new substrate for MEK reveals that the RAS/MAP kinase pathway regulates proteostasis in normal cells and that it can be targeted to promote proteomic instability and amyloidogenesis in cancer cells.