The guardian of the genome meets a viral master gene regulator at a biomolecular condensate

Abstract

As guardian of the genome, p53 exerts its tumor suppressor activity by modulating the expression of several hundreds of genes and by interacting with a large number of proteins. However, p53 can also repress viral replication and it is targeted by a variety of viral proteins to allow viral replication to proceed. p53 can repress human papillomavirus replication by binding to the viral E2 master gene regulator. Here we show how full-length p53 can spontaneously form phase separated liquid-like droplets that evolve to amyloid-like aggregates in a time-dependent manner, highlighting the fact that homotypic condensation is on the path to aggregation as observed in several protein aggregopathies. The DNA binding domain of HPV E2 (E2C) triggers heterotypic liquid-liquid phase separation with p53 with a precise 1 p53 : 2 E2C stoichiometry at the onset for demixing, yielding large regular spherical droplets that increase in size with E2C concentration. Moreover, E2C is able to slowly reshape time-evolved p53 aggregates into regular heterotypic liquid droplets. Using in situ sub-cellular fractionation, we show that E2 and wild-type p53 co-localize to the nucleus with a grainy pattern, and E2 can re-localize p53 into chromatin associated foci, a function independent of the DNA binding capacity of p53. A small DNA duplex containing the specific binding site for p53 deforms and dissolves both homotypic and heterotypic condensates at a 1 p53 : 1 DNA stoichiometry, whereas a ~1000 base pair DNA fragment instead reshaped the condensates into distinct amorphous condensates containing p53, E2C and DNA, reminiscent of what we observe bound to chromatin. We conclude that p53 is a scaffold for liquid-liquid phase separation in line with its structural and functional features, in particular as a hub that binds multiple cellular protein partners as well as nucleic acids. Moreover, the capacity of E2C to rescue p53 from the amyloid aggregation route impacts on p53-rescuing drugs cancers where p53 mutation leads to loss of function.