Abstract
The tumour suppressor protein, p16INK4A plays an important role in cell cycle regulation and is frequently mutated in human cancers. Specifically, p16INK4A inhibits cyclin dependent kinase (CDK) 4 and 6 preventing the formation of the active CDK4/6-cyclin D complex which facilitates the transition of the cell from the G1 to the S phase. The structure of p16INK4A has been shown to be crucial for its function.
Amyloid fibrils are aggregated protein structures that adopt a cross β-sheet conformation. They are commonly known for their involvement in the pathology of neurodegenerative diseases such as Alzheimer’s. Recent discoveries show that p16INK4A forms amyloids under oxidizing conditions through the formation of a disulfide dependent, dimeric building block. The absence of CDK4/6 inhibition when p16INK4A is in the amyloid state highlights that the structural state of p16INK4A is regulated by redox factors. This is an example of a unique mechanism where α-helical, monomeric proteins undergo a major structural transition into β-sheet based amyloids.
p16INK4A oxidation appears to occur in cells and this can be detected by a monoclonal antibody. This inspired the investigation of the native state of p16INK4A in a range of cancer cell lines by immunoblotting. Analysis of large p16INK4A species was performed using filter trap assays which suggested the presence of p16INK4A amyloids. The native state of p16INK4A was further investigated at different stages of the cell cycle using the double thymidine block.
Currently, it is unclear if amyloid p16INK4A species constitutes a functional state, but this study highlights the presence of a novel aggregated protein state in cultured cells. p16INK4A is the first protein identified to transition into amyloids upon oxidation. This research sets the basis for further investigation of this protein state and its potential implications in human diseases such as cancer.