The Full Complement of Yeast Ypt/Rab-GTPases and Their Involvement in Exo- and Endocytic Trafficking - Fusion of Biological Membranes and Related Problems - page 153

5, Ypt7p plays an important role in the docking step of homotypic vacuole
fusion. Antibodies to Ypt7p inhibit the docking reaction in the in vitro
vacuole fusion assay (Haas et al., 1995). The inability of the vacuoles to fuse
can be seen as the underlying cause for the vacuole fragmentation seen in
ypt7 mutant cells.
Much has been learned by studying the structure and action of Ras
proteins from animal (including human) cells. These investigations have
provided insights into which amino acids play which roles during the on/off
oscillation of the monomeric G-proteins. Most of these important amino
acids are involved in the binding of the nucleoside phosphate(s) and/or its
chemical conversion while others are known or suspected to actively con-
tribute to interactions with other proteins (see sections 2 and 3). Many of
the amino acid exchanges which effect proper functioning of Ras proteins
(Wittinghofer and Valencia, 1995) have been made for Ypt7p, too. Sur-
prisingly, many of them had no detectable detrimental effect on the
protein's functional integrity. However, a yeast strain carrying the mutant
Ypt7(T22N)p which preferentially binds GDP, displays a phenotype similar
to the ypt7 null mutant (Wada et al., 1996). A perturbation of lysosomal
transport has also been observed in an analogous mutant of Dictyostelium
discoideum (Buczynski et al., 1997). In wild-type yeast cells, the time during
which Ypt7p stays in the active, GTP-bound form appears to be controlled
through interactions with the GAP Gyp7p (Vollmer et al., 1995).
The findings from in vivo studies of ypt7 mutants and the vacuole
fusion in vitro system suggest at least two functions for Ypt7p: one in
trafficking between a late endosomal compartment and the vacuole,
and one in homotypic vacuole fusion. A role far the fission yeast homo-
logue of Ypt7p in vacuole fusion was demonstrated in vivo recently (Bone
et al., 1998). In Schizosaccharomces pombe, the vacuolar fission/fusion
process depends on the Ypt7 and Ypt4GTPases (the latter not being
present in baker's yeast) and is a physiological response to adjusting the
intracellular osmotic pressure to changing environmental conditions (Bone
et al., 1998).
6.6. Ypt6p
The cells of multicellular organisms contain a much greater variety of
Ypt proteins than single-celled microbes such as yeast. Not surprisingly, not
all of them have homologues in yeast. One of them, the mammalian Rab6,
does have homologues with an identical effector region in both baker's
and fission yeast, though: The Schizosaccharomyces pombe Ryh1p with
69% sequence identity (Hengst et al., 1990), and the S. cerevisiae Ypt6p with
75% identity (Hengst et al., 1995). YPT6 was identified and cloned using