Es. Bensen et al., Ric1p and the Ypt6p GTPase function in a common pathway required for localization of trans-Golgi network membrane proteins, MOL BIOL CE, 12(1), 2001, pp. 13-26
In Saccharomyces cerevisiae, clathrin is necessary for localization of tran
s-Golgi network (TGN) membrane proteins, a process that involves cycling of
TGN proteins between the TGN and endosomes. To characterize further TGN pr
otein localization, we applied a screen for mutations that cause severe gro
wth defects in combination with a temperature-sensitive clathrin heavy chai
n. This screen yielded a mutant allele of RIC1. Cells carrying a deletion o
f RIC1 (ric1 Delta) mislocalize TGN membrane proteins Kex2p and Vps10p to t
he vacuole. Delivery to the vacuole occurs in ric1 Delta cells also harbori
ng end3 Delta to block endocytosis, indicative of a defect in retrieval to
the TGN rather than sorting to endosomes. SYS1, originally discovered as a
multicopy suppressor of defects caused by the absence of the Rab GTPase YPT
6, was identified as a multicopy suppressor of ric1 Delta. Further comparis
on of ric1 Delta and ypt6 Delta cells demonstrated identical phenotypes. Mu
lticopy plasmids expressing v-SNAREs Gos1p or Ykt6p, but not other v- and t
-SNAREs, partially suppressed phenotypes of ric1 Delta and ypt6 Delta cells
. SLY1-20, a dominant activator of the cis-Golgi network t-SNARE Sed5p, als
o functioned as a multicopy suppressor. Because Gos1p and Ykt6p interact wi
th Sed5p, these results raise the possibility that TGN membrane protein loc
alization requires Ric1p- and Ypt6p-dependent retrieval to the cis-Golgi ne
twork.