The nuclear–cytoplasmic distribution of ERK2 is regulated in response to various stimuli and changes in cell context. Furthermore, the nuclear flux of ERK2 occurs by several energy- and carrier-dependent and -independent mechanisms. ERK2 has been shown to translocate into and out of the nucleus by facilitated diffusion through the nuclear pore, interacting directly with proteins within the nuclear pore complex, as well as by karyopherin-mediated transport. Nuclear export has been suggested to be CRM1- and MEK1/2-dependent. Here, we describe a general nuclear import assay of wild-type ERK2 that can be employed to identify different mechanisms governing nuclear entry of the protein kinase, adapted to evaluate ERK2 mutants that impair nuclear entry to dissect energy- and carrier-dependent and -independent mechanisms, and extended to characterize export mechanisms.
