The Full Complement of Yeast Ypt/Rab-GTPases and Their Involvement in Exo- and Endocytic Trafficking - Fusion of Biological Membranes and Related Problems - page 151

differential regulation of the various routes and gates. Among these found
are four out of the eleven YPT genes. They will be discussed in greater
detail below.
Ypt51p,Ypt52p and Ypt53p have been grouped together because these
isoforms share a high degree of sequence similarity and an identical effec-
tor region with mammalian Rab5 (Singer-Krüger et al., 1994). While two
members ( YPT51 (= VPS21 ) and YPT53 ) were identified in a low-
stringency PCR-screen, the sequence of YPT52 was uncovered by the total
sequencing of chromosome XI. The high degree of seqence similarity to
Rab5 suggested a similar function for the Ypt5 proteins. Heterologous
expression of Ypt51p in mammalian cells indeed resulted in a stimulation
of endocytosis and association of the GTPase with Rab5-positive endo-
somes. A constitutively activated mutant form, Ypt51(Q66L)p leads to the
same biochemical and morphological changes as the corresponding muta-
tion in the Rab5 protein (Singer-Krüger et al., 1995). Analysis of yeast
mutants deleted in one or several of the YPTS genes led to the discovery
that all the three genes are likely to play roles in delivering proteins to the
vacuole, either from the cell surface or from outside the cell by endocyto-
sis, or from the Golgi apparatus (Horazdovsky et al., 1994; Singer-Krüger et
al., 1994). Among the YPT5 group, the strongest phenotype is seen upon
deletion of the YPT51 gene. ypt51 mutants were also identified as vps21 in
a screen for yeast cells that secrete the precursor form of the vacuolar pro-
tease carboxypeptidase Y. In addititon to the protein sorting defect, the
ypt.51 phenotype shows other alterations: the acidification of the vacuoles
is disturbed and the vacuole becomes enlarged upon deletion of the YPT51
gene. The uptake of receptor-bound alpha-factor peptide is inhibited, as is
delivery to the vacuole of the dye lucifer yellow CH during internalization
by fluid-phase endocytosis. Genetic interactions with other members of the
VPS family link YPT51 and its protein product, Ypt51p, to the fusion
machinery at a prevacuolar, endosomal compartment (Burd et al., 1997;
Horazdovsky et al., 1995). Burd et al. (1997) have described genetic and
physical interactions of the t-SNARE Pepl2p and its gene at the preva-
cuolar transport step. Apart from Sec18p (NSF) and the Sec1-like protein
Vps45p, the Vac1 protein was identified. In vac1 -deletion strains, both
Pepl2p and Ypt51p show a subcellular fractionation pattern different from
wildtype cells. Along with the similar deletion phenotypes of ypt51 and
pep12 cells, this argues for a role at a neighbouring or the same vesicular
transport step. Genetic interactions were also established between
YPT51/PS21 and VPS8 . Membrane association of the RING finger
protein Vps8p, which is needed for sorting of soluble vacuolar hydrolases,
depends on Ypt51p function (Horazdovsky et al., 1996). The connection of
Ypt51p to the endocytic and vacuolar protein sorting pathways was recently