Molecular Insights into Noncanonical Histone Chaperone Complexes

Abstract

Two fundamental macromolecular complexes required for the survival of eukaryotic cells are nucleosomes, which organize our genetic material (DNA), and ribosomes, which translate the information encoded by genes into functional proteins. The protein subunits of each of these complexes must be faithfully assembled together with DNA (nucleosomes) or RNA (ribosomes) for correct function. My dissertation research focused on two factors, UBE2O and CDAN1, that interact with specific chaperones of histones, the building blocks of nucleosomes, to perform functions linked to protein complex assembly that are particularly important for red blood cell (RBC) production. In Chapter 2, I focus on how UBE2O mediates the quality control of unassembled ribosomal subunit proteins (RPs). RPs that fail to assemble into the ribosome, also called orphan RPs, must be recognized and degraded by the ubiquitin-proteasome system (UPS), a cellular protein quality control pathway. UBE2O is a unique ubiquitylation enzyme implicated in RP quality control and ribosome clearance during RBC differentiation. However, the mechanisms of specific RP client selection by UBE2O remain unclear. I describe how the histone chaperone NAP1L1 is repurposed as a substrate adaptor for UBE2O to promote ubiquitylation of orphan RP clients. Our discovery of NAP1L1 as an E3 ligase adaptor for orphan RPs represents a new noncanonical function of this histone chaperone. In Chapter 3, I focus on CDAN1, which interacts with the histone chaperone ASF1. CDAN1 is an essential protein of unknown function that has been reported to bind CDIN1, a predicted endonuclease, and ASF1. Mutations in the CDAN1 or CDIN1 genes cause a specific form of congenital dyserythropoietic anemia (CDA). Although ASF1 is required for efficient nucleosome assembly, the role of the CDAN1:ASF1 interaction and its relationship to CDIN1 and histones remains unclear. I combined structural and biochemical approaches to investigate the CDAN1:CDIN1:ASF1 (C:C:A) complex. We demonstrate that the CDAN1 complex blocks multiple functional interfaces of ASF1, including regions that recruit histones, thus forming a noncanonical complex that prevents the binding of typical ASF1 clients. We also find that CDAN1 forms a dimer capable of recruiting multiple copies of ASF1 through previously unappreciated conserved binding motifs that discriminate between the ASF1 paralogs, ASF1A and ASF1B. Altogether, my thesis research reveals new molecular insights into noncanonical histone chaperone interactors.Medical Science