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Mol. Cell. Biol., Jul 1996, 3275-3284, Vol 16, No. 7
SB Deitz, C Wu, S Silve, KE Howell, P Melancon, RA Kahn and A Franzusoff
Vesicle-mediated traffic between compartments of the yeast secretory
pathway involves recruitment of multiple cytosolic proteins for budding,
targeting, and membrane fusion events. The SEC7 gene product (Sec7p) is a
constituent of coat structures on transport vesicles en route to the Golgi
complex in the yeast Saccharomyces cerevisiae. To identify mammalian
homologs of Sec7p and its interacting proteins, we used a genetic selection
strategy in which a human HepG2 cDNA library was transformed into
conditional-lethal yeast sec7 mutants. We isolated several clones capable
of rescuing sec7 mutant growth at the restrictive temperature. The cDNA
encoding the most effective suppressor was identified as human ADP
ribosylation factor 4 (hARF4), a member of the GTPase family proposed to
regulate recruitment of vesicle coat proteins in mammalian cells. Having
identified a Sec7p-interacting protein rather than the mammalian Sec7p
homolog, we provide evidence that hARF4 suppressed the sec7 mutation by
restoring secretory pathway function. Shifting sec7 strains to the
restrictive temperature results in the disappearance of the mutant Sec7p
cytosolic pool without apparent changes in the membrane-associated
fraction. The introduction of hARF4 to the cells maintained the balance
between cytosolic and membrane-associated Sec7p pools. These results
suggest a requirement for Sec7p cycling on and off of the membranes for
cell growth and vesicular traffic. In addition, overexpression of the yeast
GTPase- encoding genes ARF1 and ARF2, but not that of YPT1, suppressed the
sec7 mutant growth phenotype in an allele-specific manner. This allele
specificity indicates that individual ARFs are recruited to perform two
different Sec7p-related functions in vesicle coat dynamics.
Copyright © 1996, American Society for Microbiology
Human ARF4 expression rescues sec7 mutant yeast cells
Department of Cellular and Structural Biology, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA.
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