Histone chaperone that facilitates histone deposition and histone exchange and removal during nucleosome assembly and disassembly. Cooperates with chromatin assembly factor 1 (CAF-1) to promote replication-dependent chromatin assembly and with HIRA to promote replication-independent chromatin assembly. Required for the formation of senescence-associated heterochromatin foci (SAHF) and efficient senescence-associated cell cycle exit. Interacts with histone H3 (including both histone H3.1 and H3.3) and histone H4. Interacts with the CHAF1A, CHAF1B and RBBP4 subunits of the CAF-1 complex. Interacts with CABIN1, HAT1, HIRA, NASP, TAF1, TLK1, TLK2 and UBN1 (By similarity). Interacts with CDAN1 (By similarity). Found in a cytosolic complex with CDAN1, ASF1B, IPO4 and histones H3.1 and H4. Interacts with CREBBP (By similarity). Phosphorylated by TLK1 and TLK2. Highly phosphorylated in S-phase and at lower levels in M-phase. TLK2-mediated phosphorylation at Ser-192 prevents proteasome-dependent degradation. Belongs to the ASF1 family.