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Vol. 9, Issue 2, 323-332, February 1998
and
*Physiological Laboratory, University of Liverpool, Liverpool, L69
3BX, United Kingdom; and
Rab5-dependent endosome fusion is sensitive to the phosphoinositide
3-kinase inhibitor, wortmannin. It has been proposed that phosphoinositide 3-kinase activity may be required for activation of
rab5 by influencing its nucleotide cycle such as to promote its active
GTP state. In this report we demonstrate that endosome fusion remains
sensitive to wortmannin despite preloading of endosomes with
stimulatory levels of a GTPase-defective mutant rab5Q79L or
of a xanthosine triphosphate-binding mutant, rab5D136N, in
the presence of the nonhydrolysable analogue XTP
Physical Biochemistry,
Max-Planck Institute, D44139, Dortmund, Germany
S. These results
suggest that activation of rab5 cannot be the principal function of the
wortmannin-sensitive factor on the endosome fusion pathway. This result
is extrapolated to all GTPases by demonstrating that endosome fusion
remains wortmannin sensitive despite prior incubation with the
nonhydrolysable nucleotide analogue GTP
S. Consistent with these
results, direct measurement of clathrin-coated vesicle-stimulated
nucleotide dissociation from exogenous rab5 was insensitive to the
presence of wortmannin. A large excess of rab5Q79L, beyond
levels required for maximal stimulation of the fusion assay, afforded
protection against wortmannin inhibition, and partial protection was
also observed with an excess of wild-type rab5 independent of GTP
S.
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