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Vol. 273, Issue 2, 793-799, January 9, 1998
,
,
From CEA-Grenoble, We characterized the in vitro fusion
of endosomal compartments from Dictyostelium discoideum.
Fusion activity was restricted to early compartments, was dependent on
cytosolic proteins, and was activated by GTP and guanosine
5
Laboratoire de Biochimie et Biophysique
des Systèmes Intégrés,
-O(3-thio)triphosphate (GTP
S). This stimulation
suggests the involvement of a small G protein, which we propose to be
Rab7 on the basis of the strong inhibitory effect of anti-Rab7
antibodies. It is noteworthy that in the presence of GTP
S, the
concentration of ATP-Mg2+ could be reduced to less than 1 nM without loss of fusion activity. Under these conditions,
competing residual ATP with adenosine 5
-O-(3-thio)triphosphate-Mg2+ also failed to
inhibit endosome fusion. The presence of an ATP-depleting system alone
blocked fusion probably because endogenous GTP was removed by coupling
through NDP kinase. Moreover, whether ATP was present or not,
GTP
S-activated fusion was equally sensitive to anti-Rab7 antibodies
or N-ethylmaleimide and was restricted to early
compartments. These results show that soluble ATP-Mg2+ is
not needed for endosome fusion. Since homotypic fusion of endosomes in
D. discoideum has been shown to depend on the ATPase N-ethylmaleimide-sensitive factor (Lenhard, J. M.,
Mayorga, L., and Stahl, P. D. (1992) J. Biol. Chem.
267, 1896-1903), the nucleotide exchange on the
N-ethylmaleimide sensitive factor must take place before
GTP
S activation in this system.
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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