RCC1, a guanine nucleotide exchange factor for Ran GTPase, plays essential roles in the growth and viability of mammalian cells. Here, we examined the phosphorylation of specific serine and threonine residues of RCC1 in vivo and showed that RCC1 is indeed phosphorylated. Analysis by two-dimensional (2D) gel electrophoresis suggested that serine 11 (S11) of hamster RCC1 is phosphorylated in vivo. A point mutation of S11 of hamster RCC1 resulted in a decrease in the number of 2D gel spots, indicating a lack of phosphorylation at the mutant residue. S11 phosphorylation in vitro depended on cyclin B-cdc2 kinase. An RCC1 mutant in which all N-terminal serine and threonine residues were substituted with glutamate residues to mimic phosphorylation at these residues showed decreased binding to the karyopherin, KPNA4, compared with wild type RCC1. We conclude that RCC1 undergoes post-translational phosphorylation.
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