RCC1 Uses a Conformationally Diverse Loop Region to Interact with the Nucleosome: A Model for the RCC1-Nucleosome Complex

The binding of RCC1 (regulator of chromosome condensation 1) to chromatin is critical for cellular processes such as mitosis, nucleocytoplasmic transport, and nuclear envelope formation because RCC1 recruits the small GTPase Ran (Ras-related nuclear protein) to chromatin and sets up a Ran-GTP gradient around the chromosomes. However, the molecular mechanism by which RCC1 binds to nucleosomes, the repeating unit of chromatin, is not known. We have used biochemical approaches to test structural models for how the RCC1 β-propeller protein could bind to the nucleosome. In contrast to the prevailing model, RCC1 does not appear to use the β-propeller face opposite to its Ran-binding face to interact with nucleosomes. Instead, we find that RCC1 uses a conformationally flexible loop region we have termed the switchback loop in addition to its N-terminal tail to bind to the nucleosome. The juxtaposition of the RCC1 switchback loop to its Ran binding surface suggests a novel mechanism for how nucleosome-bound RCC1 recruits Ran to chromatin. Furthermore, this model accounts for previously unexplained observations for how Ran can interact with the nucleosome both dependent and independent of RCC1 and how binding of the nucleosome can enhance RCC1's Ran nucleotide exchange activity.