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A more recent version of this article appeared on March 1, 2002
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Submitted on August 14, 2001
Revised on October 31, 2001
Accepted on December 20, 2001
1 Centre for Immunology, St. Vincent's Hospital, University of New South Wales, Sydney, NSW Australia
2 Discipline of Medical Biochemistry, School of Biomedical Sciences, Faculty of Medicine and Health Sciences, The University of Newcastle, Callaghan, NSW Australia
* Corresponding author. E-mail address: r.ludowyke{at}cfi.unsw.edu.au.
Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5 fold) in these proteins. PMA plus A23187 induce the same changes, at a time coincident with its slower rate of secretion. Co-immunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements and led to increased phosphorylation of the myosin heavy and light chains at PKC-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A is required for mast cell secretion.
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