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Department of Cellular and Structural Biology, University of Colorado School of Medicine, Denver, Colorado 80262
An 85-kD cytosolic complex (p62cplx), consisting of a 62-kD phosphoprotein (p62) and a 25-kD
GTPase, has been shown to be essential for the cell-free
reconstitution of polymeric IgA receptor (pIgA-R)-containing exocytic transport vesicle formation from the
TGN (Jones, S.M., J.R. Crosby, J. Salamero, and K.E.
Howell. 1993. J. Cell Biol. 122:775-788). Here the p62cplx
is identified as a regulatory subunit of a novel phosphatidylinositol 3-kinase (PI3-kinase). This p62cplx-associated PI3-kinase activity is stimulated by activation of the p62cplx-associated GTPase, and is specific for phosphatidylinositol (PI) as substrate, and is sensitive to
wortmannin at micromolar concentrations. The direct
role of this p62cplx-associated PI3-kinase activity in
TGN-derived vesicle formation is indicated by the finding that both lipid kinase activity and the formation of
pIgA-R-containing exocytic vesicles from the TGN are
inhibited by wortmannin with similar dose-response
curves and 50% inhibitory concentrations (3.5 µM).
These findings indicate that phosphatidylinositol-3-phosphate (PI[3]P) is required for the formation of TGN-derived exocytic transport vesicles, and that the p62cplx-associated PI3-kinase and an activated GTPase are the
essential molecules that drive production of this PI(3)P.
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