The mitogen-activated protein (MAP) kinases ERK1 and ERK2 often accumulate in the nuclei of stimulated cells to mediate changes in transcription. The mechanisms underlying stimulus-dependent redistribution of these kinases remain unclear. We have used a permeabilized cell reconstitution assay in HeLa cells and human foreskin fibroblasts to explore the processes by which ERK2 enters and exits the nucleus. We previously reported that entry of unphosphorylated ERK2 into the nucleus occurs by facilitated diffusion not requiring cytosolic transport factors. We find that export, like import, can occur by an energy- and carrier-independent mechanism. An energy-dependent mechanism of ERK2 export can also be distinguished, mediated at least in part through the exportin CRM1. We have also examined import and export of thiophosphorylated, active ERK2. Import of active ERK2 is significantly enhanced by the addition of exogenous transport factors and an energy regeneration system. These studies support a model in which multiple constitutive and regulated processes control the subcellular distribution of ERK2.
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