One Chaperone to Rule Them All: Deciphering How Chromatin is Assembled During DNA Replication

Abstract

Genomic DNA, which governs cellular life, resides within the nucleus of every human cell. Inside each nucleus lies approximately two meters of DNA, posing a significant challenge, akin to compacting 20 kilometers of thread into a tennis ball. To overcome this challenge, DNA is packaged by wrapping itself around proteins called histones, forming individual structures known as nucleosomes. This succession of nucleosomes creates an organization that resembles beads on string, commonly referred to as chromatin. Histones not only serve as a structural framework for the chromatin; they also regulate its accessibility by carrying chemical modifications that directly impact vital cellular processes. Preserving this chromatin information, also referred to as epigenetic information, is crucial to ensuring the survival of each cell and, by extension, the organism. Throughout life, many cell types in the human body undergo cell divisions (like skin cells for instance). However, before any cell division can occur, the cell must replicate its genetic content to provide an intact copy of the genome to the future two daughter cells. This process is known as DNA replication. Alongside DNA replication, epigenetic information on histones is meticulously reconstituted, via a process called chromatin replication. Failure in chromatin replication would severely affect the function and identity of the future daughter cells. For instance, a skin cell that failed to replicate its chromatin before dividing, would give rise to two daughter cells that are no longer skin cells. Chromatin replication involves the wrapping of DNA around histones to form chromatin. To ensure the smooth progress of this operation, histone proteins, known for their instability and propensity for aggregation, require careful handling. Histone chaperones, a class of proteins, take charge of protecting histones, ensuring their controlled trafficking, and facilitating their deposition onto the DNA. In the context of chromatin replication, the histone chaperone Chromatin Assembly Factor 1 (CAF-1) plays a central role by directly depositing histones onto the DNA. CAF-1 is precisely targeted to DNA synthesis sites through direct interaction with the protein PCNA. The work presented in this thesis explores the complex coordination between CAF-1 and PCNA in chromatin replication by examining the dynamics of their interaction during DNA synthesis.