Acts as a dual histone chaperone and heat shock co-chaperone (By similarity). 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 (By similarity). May also act as a histone co-chaperone together with TONSL (By similarity). May recruit histone chaperones ASF1A, NASP and SPT2 to histone H3-H4 heterodimers (By similarity). Also plays a role as co-chaperone of the HSP70 family of molecular chaperone proteins, such as HSPA1A, HSPA1B and HSPA8 (By similarity). 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 (By similarity). Exhibits activity to assemble histones onto DNA in vitro (By similarity). Forms a co-chaperone complex with MCM2 and histone H3.3-H4 dimers (By similarity). Within the complex, interacts (via C-terminus) with MCM2 (via N-terminus); the interaction is histone-dependent (By similarity). Within the complex, interacts (via C-terminus) with histone H3.3-H4 heterodimers; the interaction is direct (By similarity). Interacts with histones H4, H3.3, H3.2 and H3.1, but not with CENPA or the testis-specific histone H3.1t (By similarity). Interacts (via J domain) with HSPA1A, HSPA1B and HSPA8 (By similarity). May interact with TONSL; the interaction seems to be histone-dependent (By similarity). May interact with HSPA8 and BAG2; the interactions seem to be histone-dependent (By similarity). Predominantly nuclear. Translocates to the cytoplasm and membrane after heat shock. The functional J domain is required for the release from histone-dependent chromatin-binding.