Proper genome packaging requires coordination of both DNA and histone metabolism. While histone gene transcription and RNA processing adequately provide for scheduled needs, how histone supply adjusts to unexpected changes in demand remains unknown. Here, we reveal that the histone chaperone nuclear autoantigenic sperm protein (NASP) protects a reservoir of soluble histones H3-H4. The importance of NASP is revealed upon histone overload, engagement of the reservoir during acute replication stress, and perturbation of Asf1 activity. The reservoir can be fine-tuned, increasing or decreasing depending on the level of NASP. Our data suggest that NASP does so by balancing the activity of the heat shock proteins Hsc70 and Hsp90 to direct H3-H4 for degradation by chaperone-mediated autophagy. These insights into NASP function and the existence of a tunable reservoir in mammalian cells demonstrate that contingency is integrated into the histone supply chain to respond to unexpected changes in demand.
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