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PLANT PHYSIOLOGY , Vol 112, Issue 1 421-432, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Cytoplasmic Orientation of the Naphthylphthalamic Acid-Binding Protein in Zucchini Plasma Membrane Vesicles
M. W. Dixon, J. A. Jacobson, C. T. Cady and G. K. Muday
Wake Forest University, Department of Biology, Box 7325, Winston-Salem, North Carolina 27109-7325 (M.W.D., G.K.M.)
Polar transport of the plant hormone auxin is blocked by substances such as
N-1-naphthylphthalamic acid (NPA), which inhibit auxin efflux and block
polar auxin transport. To understand how auxin transport is regulated in
vivo, it is necessary to discern whether auxin transport inhibitors act at
the intra- or extracellular side of the plasma membrane. Populations of
predominantly in-side-in plasma membrane vesicles were subjected to
treatments that reverse the orientation. These treatments, which included
osmotic shock, cycles of freezing and thawing, and incubation with 0.05%
Brij-58, all increased NPA-binding activity and the accessibility of the
binding protein to protease digestion. Marker activities for inside-out
vesicles also increased, indicating that these treatments act by altering
the membrane orientation. Finally, binding data were analyzed by multiple
analyses and indicated that neither the affinity nor abundance of binding
sites changed. Kinetic analyses indicated that the change in NPA-binding
activity by Brij-58 treatment was due to an increase in the initial rates
of both association and dissociation of this ligand. These experiments
indicated that the NPA-binding site is on the cytoplasmic face of the
plasma membrane in zucchini (Cucurbita pepo L. cv Burpee Fordhook).
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