Acts as a dual histone chaperone and heat shock co-chaperone. 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. May also act as a histone co-chaperone together with TONSL. May recruit histone chaperones ASF1A, NASP and SPT2 to histone H3-H4 heterodimers. Also plays a role as co-chaperone of the HSP70 family of molecular chaperone proteins, such as HSPA1A, HSPA1B and HSPA8. 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. Exhibits activity to assemble histones onto DNA in vitro.