Tumour suppressors are proteins that are critical in preventing the development of cancers. They protect the cell by blocking cell division and cell growth, and they are therefore frequently mutated in all types of cancers. One important family of tumour suppressors is the INK4 inhibitors. p18INK4c is a tumour suppressor protein of the INK4 family that is associated with many blood and brain cancers. p18INK4c specifically inhibits cyclin-dependent kinase (CDK) 4/6-cyclin-D complexes to arrest cell division.
We recently discovered a novel mechanism affecting the function of p18INK4c. Under oxidative conditions, p18INK4c undergoes homo-dimerization through a disulfide bridge formed via its single cysteine residue (C141). The all α-helical protein transforms into inactive amyloid fibrils with a cross β-sheet structure following this dimerization. Amyloid fibrils, are known for their association with neurodegenerative diseases, but have recently been linked to a cancer-related proteins. This discovery highlights how oxidation can induce major structural changes in tumour suppressors, impacting their functionality. By studying the recombinantly produced protein, we have revealed that the transition occurs under mild oxidizing conditions at physiological pH and temperature. We further studied the p18INK4C transition in HEK293 cells. We find that oxidation leads to a rapid conversion into aggregates of both, endogenous and ectopically overexpressed protein by using fluorescence microscopy. Furthermore, cancer-associated mutations in p18INK4c enhance its propensity to form amyloid fibrils.
Our study introduces a novel concept in tumour biology, emphasizing the existence of different structural and functional states in the INK4 family of tumour suppressors and expanding our understanding of their role in cancer. Furthermore, the loss of inhibitor function of p18INK4c upon the formation of an irreversible amyloid fibril state suggests a potential functional role for amyloids in its activity. These non-functional amyloid fibrils may be utilized by the cell to facilitate progression through the cell cycle.