early in vivo studies demonstrated the involvement of a tumor-suppressing transcription factor, p53, into cellular droplets such as Cajal and promyelocytic leukemia protein bodies, suggesting that the liquid-liquid phase separation (LLPS) might be involved in the cellular functions of p53. To examine this possibility, we conducted extensive investigations on the droplet formation of p53 in vitro. First, p53 itself was found to form liquid-like droplets at neutral and slightly acidic pH and at low salt concentrations. Truncated p53 mutants modulated droplet formation, suggesting the importance of multivalent electrostatic interactions among the N-terminal and C-terminal domains. Second, FRET efficiency measurements for the dimer mutants of p53 revealed that distances between the core domains and between the C-terminal domains were modulated in an opposite manner within the droplets. Third, the molecular crowding agents were found to promote droplet formation, whereas ssDNA, dsDNA, and ATP, to suppress it. Finally, the p53 mutant mimicking posttranslational phosphorylation did not form the droplets. We conclude that p53 itself has a potential to form droplets that can be controlled by cellular molecules and by posttranslational modifications, suggesting that LLPS might be involved in p53 function. Tumor suppressor p53 is a multifunctional transcription factor that induces cell cycle arrest, DNA repair or apoptosis upon binding to its target DNA sequence. In 50% of human cancers, mutations on p53 are found to hamper its binding to the target sequence. Accordingly, extensive investigations have been conducted to characterize the functions as well as malfunctions of p53. However, an important aspect of p53, namely its involvement in liquid-like droplets, is still largely unresolved. In fact, p53 has long been known to be uptaken into cellular droplets such as Cajal and promyelocytic leukemia protein (PML) bodies. In this report, we describe that p53 itself can form liquid-like droplets upon the control of solution conditions, suggesting a possible involvement of the p53 droplets in the cellular environment. The primary function of p53 is the accommodation of various posttranslational modifications, termed activation, which in turn triggers the search for and the binding to its target DNA sequence, leading to the expression of downstream genes 1. p53 is composed of the N-terminal (NT) (residues 1-95), the core (95-293), the linker (293-326), the tetramerization (Tet) (326-357), and the C-terminal (CT) (357-393) domains. p53 slides along nonspecific DNA by attaching the CT domain to the DNA and by hopping the core domain 2-4. The sliding of p53 occurs in two modes 5,6 , in which the CT, core and linker domains are differently in contact with the DNA 7. The recognition efficiency of the target sequence by the sliding p53 is low, but is enhanced by the activation of p53 8. Furthermore, the sliding p53 can transfer from one DNA strand to another using the CT domain 9. In addition,
