Regulator of chromosome condensation (RCC1) binding to chromatin is highly dynamic, as determined by fluorescence recovery after photobleaching analysis of GFP-RCC1 in stably transfected tsBN2 cells. Microinjection of wild-type or Q69L Ran markedly slowed the mobility of GFP-RCC1, whereas T24N Ran (defective in nucleotide loading) decreased it further still. We found significant alterations in the mobility of intranuclear GFP-RCC1 after treatment with agents that disrupt different Ran-dependent nuclear export pathways. Leptomycin B, which inhibits Crm1/RanGTP-dependent nuclear export, significantly increased the mobility of RCC1 as did high levels of actinomycin D (to inhibit RNA polymerases I, II, and III) or alpha-amanitin (to inhibit RNA polymerases II and III) as well as energy depletion. Inhibition of just mRNA transcription, however, had no affect on GFP-RCC1 mobility consistent with mRNA export being a Ran-independent process. In permeabilized cells, cytosol and GTP were required for the efficient release of GFP-RCC1 from chromatin. Recombinant Ran would not substitute for cytosol, and high levels of supplemental Ran inhibited the cytosol-stimulated release. Thus, RCC1 release from chromatin in vitro requires a factor(s) distinct from, or in addition to, Ran and seems linked in vivo to the availability of Ran-dependent transport cargo.
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