Investigating the structure of wild-type and HP1-bound human chromatin by cryoEM

Abstract

In the field of chromatin biology, there are still many unknowns in regards to the true higher-order structure of chromatin. One of the main models is the ‘two-start’ model, suggesting an open zig-zag structure of parallel nucleosome stacks twists into a helical 30-nm fiber. With advancements in cryo-EM, we are in a position to solve high-resolution chromatin structures to investigate this model further. An understanding of wild-type unmodified chromatin structure is essential for elucidating the role of various chromatin regulatory proteins. Heterochromatin Protein 1 (HP1) is a major player in regulating heterochromatin formation and repressing gene expression, and is heavily implicated in disease. In this project, we utilised biochemical and structural biology techniques to elucidate structures of 12-nucleosome arrays under physiological salt conditions. Wild-type arrays demonstrated a flat structure of two parallel nucleosome stacks, with a high degree of flexibility. Addition of HP1 isoforms to nucleosome arrays revealed differences between the three human isoforms, suggesting different roles in vivo. Structural studies have suggested that HP1α promotes stabilisation of the twisted form of the 12-mer array. Overall this project has opened several exciting avenues for future studies and provided vital insight into chromatin structure at the most fundamental level.