© The Rockefeller University Press, 0021-9525/2000/4/331/ $5.00
The Journal of Cell Biology, Volume 149, Number 2, April 17, 2000 331-340
A Specific Activation of the Mitogen-activated Protein Kinase Kinase 1 (MEK1) Is Required for Golgi Fragmentation during Mitosis
Antonino Colanzia,
Thomas J. Deerinckb,
Mark H. Ellismanb, and
Vivek Malhotraa
a Department of Biology, Department of Neurosciences, University of California San Diego, La Jolla, California 92093-0347
b National Center for Microscopy and Imaging Research, Department of Neurosciences, University of California San Diego, La Jolla, California 92093-0347
Correspondence to:
Vivek Malhotra, Department of Biology, B0347, University of California San Diego, Pacific Hall, Room 2222A, La Jolla, CA 92093-0347. Tel:(858) 534-8910 Fax:(858) 534-0555 E-mail:malhotra{at}biomail.ucsd.edu.
Incubation of permeabilized cells with mitotic extracts results in extensive fragmentation of the pericentriolarly organized stacks of cisternae. The fragmented Golgi membranes are subsequently dispersed from the pericentriolar region. We have shown previously that this process requires the cytosolic protein mitogen-activated protein kinase kinase 1 (MEK1). Extracellular signal–regulated kinase (ERK) 1 and ERK2, the known downstream targets of MEK1, are not required for this fragmentation (Acharya et al. 1998 
). We now provide evidence that MEK1 is specifically phosphorylated during mitosis. The mitotically phosphorylated MEK1, upon partial proteolysis with trypsin, generates a different peptide population compared with interphase MEK1. MEK1 cleaved with the lethal factor of the anthrax toxin can still be activated by its upstream mitotic kinases, and this form is fully active in the Golgi fragmentation process. We believe that the mitotic phosphorylation induces a change in the conformation of MEK1 and that this form of MEK1 recognizes Golgi membranes as a target compartment. Immunoelectron microscopy analysis reveals that treatment of permeabilized normal rat kidney (NRK) cells with mitotic extracts, treated with or without lethal factor, converts stacks of pericentriolar Golgi membranes into smaller fragments composed predominantly of tubuloreticular elements. These fragments are similar in distribution, morphology, and size to the fragments observed in the prometaphase/metaphase stage of the cell cycle in vivo.
Key Words:
mitogen-activated protein kinase kinase 1, extracellular signal–regulated kinases, mitosis, Golgi fragmentation, phosphorylation
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