RCC1 isoforms differ in their affinity for chromatin, molecular interactions and regulation by phosphorylation

RCC1 is the guanine nucleotide exchange factor for Ran GTPase. Generation of Ran-GTP by RCC1 on chromatin provides a spatial signal that directs nucleocytoplasmic transport, mitotic spindle assembly and nuclear envelope formation. We show that RCC1 is expressed in human cells as at least three isoforms, named RCC1a, RCC1ß and RCC1?, which are expressed at different levels in specific tissues. The ß and ? isoforms contain short inserts in their N-terminal regions (NTRs) that are not present in RCC1a. This region mediates interaction with chromatin, binds importin a3 and/or importin ß, and contains regulatory phosphorylation sites. RCC1? is predominantly localised to the nucleus and mitotic chromosomes like RCC1a. However, compared to RCC1a, RCC1? has a greatly reduced interaction with an importin a3-ß and a stronger interaction with chromatin that is mediated by the extended NTR. RCC1? is also the isoform that is most highly phosphorylated at serine 11 in mitosis. Unlike RCC1a, RCC1? supports cell proliferation in tsBN2 cells more efficiently when serine 11 is mutated to non-phosphorylatable alanine. Phosphorylation of RCC1? therefore specifically controls its function during mitosis. These results show that human RCC1 isoforms have distinct chromatin binding properties, different molecular interactions, and are selectively regulated by phosphorylation, as determined by their different NTRs.