Structural insights into the interaction of Hir2 and Hpc2 in the yeast Hir histone chaperone complex

The HIRA complex, composed of HIRA, UBN1, and CABIN1 in humans, plays a central role in histone chaperone activity and chromatin regulation by depositing the H3.3 histone variant into nucleosomes. Proper subunit interactions are critical for complex stability and function. In this study, we examine the interaction between Hir2 and Hpc2, the yeast homologs of HIRA and UBN1, using biochemical and structural approaches. We show that the N-terminal to the Hpc2-related domain (NHRD) of Hpc2 binds to the WD40 domain of Hir2, consistent with the human HIRA-UBN1 interaction. The crystal structure of the Hir2_WD40-Hpc2_NHRD complex reveals a seven-bladed β-propeller fold in Hir2_WD40, with Hpc2_NHRD forming an antiparallel β sheet interface. Notably, a unique five-stranded blade in Hir2_WD40, stabilized by proline residue P228, is essential for Hpc2 binding. Mutational analysis confirms key interface residues, providing structural insights into the evolutionary conservation of the HIRA complex.