Acts as a dual histone chaperone and heat shock co-chaperone (PubMed:33857403). As a histone chaperone, forms a co-chaperone complex with MCM2 and histone H3-H4 heterodimers; and may thereby assist MCM2 in histone H3-H4 heterodimer recognition and facilitate the assembly of histones into nucleosomes (PubMed:33857403). May also act as a histone co-chaperone together with TONSL (PubMed:33857403). May recruit histone chaperones ASF1A, NASP and SPT2 to histone H3-H4 heterodimers (PubMed:33857403). Also plays a role as co-chaperone of the HSP70 family of molecular chaperone proteins, such as HSPA1A, HSPA1B and HSPA8 (PubMed:17182002, PubMed:33857403). As a co-chaperone, may play a role in the recruitment of HSP70-type molecular chaperone machinery to histone H3-H4 substrates, thereby maintaining the histone structural integrity (PubMed:33857403). Exhibits activity to assemble histones onto DNA in vitro (PubMed:33857403). Forms a co-chaperone complex with MCM2 and histone H3.3-H4 heterodimers (PubMed:33857403). Within the complex, interacts (via C-terminus) with MCM2 (via N-terminus); the interaction is histone-dependent (PubMed:33857403). Within the complex, interacts (via C-terminus) with histone H3.3-H4 heterodimers; the interaction is direct (PubMed:33857403). Interacts with histones H4, H3.3, H3.2 and H3.1, but not with CENPA or the testis-specific histone H3.1t (PubMed:33857403). Interacts (via J domain) with HSPA1A, HSPA1B and HSPA8 (PubMed:17182002, PubMed:33857403). May interact with TONSL; the interaction seems to be histone-dependent (PubMed:33857403). May interact with HSPA8 and BAG2; the interactions seem to be histone-dependent (PubMed:33857403). Predominantly nuclear. Translocates to the cytoplasm and membrane after heat shock. Expressed in heart, placenta, liver, skeletal muscle, kidney, pancreas, thymus, ovary, colon and peripheral blood. By heat shock, bacterial lipopolysaccharides (LPS), phorbol 12-myristate 13-acetate (PMA), and the cytokine TNF (at protein level). The functional J domain is required for the release from histone-dependent chromatin-binding.