Cytoplasmic localization of mitogen-activated protein kinase kinase directed by its NH2-terminal, leucine-rich short amino acid sequence, which acts as a nuclear export signal.

Mitogen-activated protein kinase (MAPK) is activated in cytoplasm in response to extracellular signals and then is translocated to nucleus. A directed activator for MAPK, MAPK kinase (MAPKK), stays in cytoplasm to transmit the signal from the plasma membrane to MAPK. Here we show that MAPKK contains a short amino acid sequence in the N-terminal region (residues 32-44), which acts as a nuclear export signal (NES) and thus is required for cytoplasmic localization of MAPKK. This NES sequence of MAPKK, like that of protein kinase inhibitor of cAMP-dependent protein kinase or Rev, is rich in leucine residues, which are crucial for the NES activity. Furthermore, the NES peptide of protein kinase inhibitor, as well as the NES peptide of MAPKK, inhibited the nuclear export of ovalbumin conjugated to the NES peptide of MAPKK. These results may suggest a common mechanism of nuclear export using a general leucine-rich NES.