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J. Cell Biol.,
Volume 142, Number 3, August 10, 1998 803-813
* Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 and We used melanophores, cells specialized for
regulated organelle transport, to study signaling pathways involved in the regulation of transport. We transfected immortalized Xenopus melanophores with plasmids encoding epitope-tagged inhibitors of protein
phosphatases and protein kinases or control plasmids
encoding inactive analogues of these inhibitors. Expression of a recombinant inhibitor of protein kinase A
(PKA) results in spontaneous pigment aggregation. Cold
Spring Harbor Laboratory, Cold Spring Harbor, New York 11724
-Melanocyte-stimulating hormone (MSH), a stimulus
which increases intracellular cAMP, cannot disperse
pigment in these cells. However, melanosomes in these
cells can be partially dispersed by PMA, an activator of
protein kinase C (PKC). When a recombinant inhibitor of PKC is expressed in melanophores, PMA-induced
pigment dispersion is inhibited, but not dispersion induced by MSH. We conclude that PKA and PKC activate two different pathways for melanosome dispersion. When melanophores express the small t antigen of
SV-40 virus, a specific inhibitor of protein phosphatase
2A (PP2A), aggregation is completely prevented. Conversely, overexpression of PP2A inhibits pigment dispersion by MSH. Inhibitors of protein phosphatase 1 and protein phosphatase 2B (PP2B) do not affect pigment movement. Therefore, melanosome aggregation
is mediated by PP2A.
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