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 RCC1 alpha, RCC1 beta and RCC1 gamma, which are expressed at different levels in specific tissues. The beta and gamma isoforms contain short inserts in their N-terminal regions ( NTRs) that are not present in RCC1 alpha. This region mediates interaction with chromatin, binds importin alpha 3 and/or importin beta, and contains regulatory phosphorylation sites. RCC1 gamma is predominantly localised to the nucleus and mitotic chromosomes like RCC1 alpha. However, compared to RCC1 alpha, RCC1 gamma has a greatly reduced interaction with an importin alpha 3-beta and a stronger interaction with chromatin that is mediated by the extended NTR. RCC1 gamma is also the isoform that is most highly phosphorylated at serine 11 in mitosis. Unlike RCC1 alpha, RCC1 gamma supports cell proliferation in tsBN2 cells more efficiently when serine 11 is mutated to non-phosphorylatable alanine. Phosphorylation of RCC1 gamma 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.