Unveiling the Conformational Dynamics of the Histone Tails Using Markov State Modeling

Biomolecules predominantly exert their function by altering conformational dynamics. The nucleosome core particle (NCP) is the fundamental unit of chromatin. DNA with ∼146 base pairs wraps around the histone octamer to form a nucleosome. The histone octamer is composed of two copies of each histone protein (H3, H4, H2A, and H2B) with a globular core and disordered N-terminal tails. Epigenetic modifications of the histone N-terminal tails play a critical role in regulating the chromatin structure and biological processes such as transcription and DNA repair. Here, we report all-atom molecular dynamics (MD) simulations of the nucleosome at microsecond time scales to construct Markov state models (MSMs) to elucidate distinct conformations of the histone tails. We employ time-lagged independent component analysis (tICA) to capture their essential slow dynamics, with k-means clustering used to discretize the conformational space. MSMs unveil distinct states and transition probabilities to characterize the dynamics and kinetics of the tails. Next, we focus on the H2B tail, which is one of the least studied tails. We show that acetylation increases secondary structure formation with increased transition rates. These findings will aid in understanding the functional implications of tail conformations for nucleosome stability and gene regulation.