Acts as histone chaperone in nucleosome assembly (PubMed:28366643). In condensing spermatids, mediates the loading of the heterodimer composed of histones H2AB1 and H2BC1/TH2B onto the nucleosomes, thereby promoting the replacement of histones to protamine in male germ cells (PubMed:28366643). Interacts with core (H2A, H2B, H3, H4) and linker (H1) histones. 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. Phosphorylated form localizes in the cytoplasm during the G0/G1 transition, whereas dephosphorylation leads to relocalization into the nucleus at the G1/S-boundary. Polyglutamylated and polyglycylated. These 2 modifications occur exclusively on glutamate residues and result in either polyglutamate or polyglycine chains on the gamma-carboxyl group. Both modifications can coexist on the same protein on adjacent residues, and lowering polyglycylation levels increases polyglutamylation, and reciprocally. Polyglutamylated by TTLL4. 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. Belongs to the nucleosome assembly protein (NAP) family.