Histones hold on to RCC1

Abstract

RCC1 is the GDP–GTP exchange factor for the small GTPase Ran, which is involved in several essential cellular processes, including nucleocytoplasmic transport, nuclear envelope assembly and mitotic spindle formation. RCC1 was first discovered as a ṟegulator of c̱hromosome c̱ondensation and has long been known to bind to chromatin. The N-terminal domain of RCC1 can bind to DNA in vitro, but this domain is dispensable for in vivo association of RCC1 with chromatin. Nemergut et al.1xChromatin docking and exchange activity enhancement of RCC1 by histones H2A and H2B. Nemergut, M.E. et al. Science. 2001; 292: 1540–1543Crossref | PubMed | Scopus (147)See all References1 have now discovered the clue to the chromatin localization of RCC1, demonstrating that RCC1 binds directly to histones H2A and H2B and, furthermore, that this binding stimulates the nucleotide exchange activity of RCC1.Nemergut et al.1xChromatin docking and exchange activity enhancement of RCC1 by histones H2A and H2B. Nemergut, M.E. et al. Science. 2001; 292: 1540–1543Crossref | PubMed | Scopus (147)See all References1 first showed that RCC1 copurified with mononucleosomes, then proceeded to demonstrate direct, specific in vitro binding of RCC1 to the core domains of histones H2B and H2A. Histone-bound RCC1 is able to perform its catalytic activity and, in fact, H2A/H2B specifically enhanced RCC1-mediated nucleotide exchange by increasing the dissociation rate constant of the reaction.The effect of H2A/H2B binding on RCC1 nucleotide exchange activity strongly implied an in vivo functional relevance for the interaction, but, to test this directly, the authors took advantage of two unique features of Xenopus sperm chromatin. Chromatin isolated from Xenopus sperm contains sperm-specific proteins instead of histones H2A/H2B, and intriguingly Xenopus sperm are the only cells in which RCC1 is not primarily chromatin associated. The addition of egg lysate to sperm chromatin triggers two sequential decondensation reactions, the first of which involves the nucleoplasmin-catalyzed replacement of the sperm-specific proteins with H2A/H2B. In this system, RCC1–GFP showed strong association with sperm chromatin only when incubated with both H2A/H2B and nucleoplasmin, confirming an in vivo role for H2A/H2B in RCC1 localization.The localization of RCC1 to chromatin creates a Ran–GTP gradient that directs growth of mitotic spindles in the proper orientation and is also an essential component of nucleocytoplasmic transport. Binding of RCC1 to H2A/H2B thus anchors this crucial Ran regulatory factor such that a stable Ran–GTP gradient is maintained throughout the cell cycle.