Nuclear localization of mitogen-activated protein kinase kinase 1 (MKK1) is promoted by serum stimulation and G2-M progression. Requirement for phosphorylation at the activation lip and signaling downstream of MKK.

Stimulation of mammalian cells results in subcellular relocalization of Ras pathway enzymes, in which extracellular signal-regulated protein kinases rapidly translocate to nuclei. In this study, we define conditions for nuclear localization of mitogen-activated protein kinase kinase 1 (MKK1) by examining effects of perturbing the nuclear export signal (NES), the regulatory phosphorylation sites Ser218 and Ser222, and a regulatory domain at the N terminus. After disrupting the NES (Delta32-37), nuclear uptake of MKK was enhanced when quiescent cells were activated with serum-phorbol 12-myristate 13-acetate or BXB-Raf-1 cotransfection. Uptake was enhanced by mutation of Ser218 and Ser222 to Glu and Asp, respectively, and blocked by mutation of these residues to Ala, although mutation of Lys97 to Met, which renders MKK catalytically inactive, did not interfere with uptake. Therefore, nuclear uptake of MKK requires incorporation of phosphate or negatively charged residues at the activation lip but not enzyme activity. On the other hand, uptake of an active MKK mutant with disrupted NES (Delta32-51) was elevated in quiescent as well as stimulated cells, and pretreatment of cells with the MKK inhibitor 1,4-diamino-2, 3-dicyano-1,4-bis[2-aminophenylthio]butadiene blocked nuclear uptake. Thus, signaling downstream of MKK is also necessary for translocation. Finally, wild type MKK containing an intact NES translocates to nuclei during mitosis before envelope breakdown. Comparison of mutants with Ser to Glu and Asp or Ala substitutions indicates that Ser phosphorylation is also required for mitotic nuclear uptake of MKK.