Autophagy is
a vacuolar/lysosomal sequestrating process induced by nutrient
starvation. Our laboratory has isolated genes essential for autophagy
(termed APG, Autophagy), and we have been
investigating the function of the gene products. To our surprise, Apg
proteins are also employed under vegetative growth conditions for the
cytoplasm to vacuole targeting (Cvt) pathway, another vesicular
transport in Saccharomyces cerevisiae. The Cvt pathway is
biosynthetic, delivering a resident hydrolase, aminopeptidase I (API),
to the vacuole. Induction of autophagy is repressed by Tor protein, a
phosphatidylinositol kinase-related kinase. Here we show that Tor
negatively regulates Apg1, a protein kinase whose activation is
necessary for autophagy. And we also found that Apg1-associating
protein, Apg13 is required for Apg1 activation in response to nutrient
starvation and rapamycin treatment. Activation of Apg1 occurs rapidly
after rapamycin treatment. And Apg1-Apg13 association is required for
autophagy but not for the Cvt pathway, suggesting this association is
regulated by nutrient condition via Tor proteins. Furthermore
additional Apg1-associating proteins Apg17 and Cvt9 function
specifically in autophagy or the Cvt pathway, respectively, suggesting
that the Apg1 complex plays an important role in switching between two
distinct vesicular transport pathways in response to nutrient
conditions.
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