Histone chaperone that facilitates histone deposition and histone exchange and removal during nucleosome assembly and disassembly (By similarity). While encoded by a region of the Arabidopsis thaliana genome that is homologous to the Brassica S-locus for self incompatibility, this protein may not play the same role in Arabidopsis thaliana. Interacts with histone H3 and histone H4 (By similarity). Component of the HIRA complex made of UBN1, UBN2, ASF1A, CABIN1 and HIRA. Interacts with HIRA (PubMed:25086063). Localized at rDNA loci in the nucleolus. Expressed in leaves and flower buds. Belongs to the ASF1 family.