Acts as histone chaperone in nucleosome assembly. Interacts with core (H2A, CD2APH2B, H3, H4) and linker (H1) histones. (Microbial infection) Interacts with Chikungunya virus non-structural protein 3 (via C-terminus). Present in the cytoplasm and excluded from the nucleus during G0/G1 phase, then relocates to the nucleus by the time cells are in S phase (PubMed:9325046). Phosphorylated form localizes in the cytoplasm during the G0/G1 transition, whereas dephosphorylation leads to relocalization into the nucleus at the G1/S-boundary (PubMed:10764593). Ubiquitous. Biallelically expressed in fetal and adult tissues. Highest levels in testis. Phosphorylated at the G0/G1 boundary but it is not phosphorylated in S-phase. Phosphorylated protein remains in the cytoplasm in a complex with histones during the G0/G1 transition, whereas dephosphorylation triggers its transport into the nucleus at the G1/S-boundary. Polyglutamylated by TTLL4, a modification that occurs exclusively on glutamate residues and results in polyglutamate chains on the gamma-carboxyl group. Some residues may also be monoglycylated but not polyglycylated due to the absence of functional TTLL10 in human (By similarity). Belongs to the nucleosome assembly protein (NAP) family.