Multiple, Nutrient Sensing Kinases Converge to Phosphorylate an Element of cdc34 That Increases Saccharomyces Cerevisiae Lifespan

Abstract

Growth and division are tightly coordinated with available nutrient conditions. Cells of the budding yeast, Saccharomyces cerevisiae, grow to a larger size prior to budding and DNA replication when preferred carbon sources such as glucose, as opposed to less preferred sources like ethanol and acetate, are available. A culture’s doubling time is also significantly reduced when the available carbon and nitrogen sources are more favorable. These physiological phenomena are well documented but the precise molecular mechanisms relaying nutrient conditions to the growth and division machinery are not well defined. I demonstrate here that Cdc34, the ubiquitin conjugating enzyme that promotes S phase entry, is phosphorylated upon a highly conserved serine residue which is part of a motif that defines the family of Cdc34/Ubc7 ubiquitin conjugating enzymes. This phosphorylation is regulated by multiple, nutrient sensing kinases including Protein Kinase A, Sch9 and TOR. Furthermore, this phosphorylation event is regulated through the cell cycle with the sole induction occurring in the G1 phase which is when nutrients are sensed and cells commit to another round of division. This phosphorylation likely activates Cdc34 and in turn propagates a signal to the cell division cycle machinery that nutrient conditions are favorable for commitment to a new round of division. This phosphorylation is critical for normal cell cycle progression but must be carefully controlled when cells are deprived of nutrients. Crippling the activity of Protein Kinase A, SCH9 or TOR increases the proportion of cells that survive stationary phase conditions, which because of the metabolic conditions that must be maintained and the similarity to post-mitotic mammalian cells, is referred to as a yeast culture’s chronological lifespan. Yeast cells expressing Cdc34 mutants that are no longer subject to this regulation by phosphorylation have a reduced chronological lifespan. A precise molecular mechanism describing the change in Cdc34 activity after phosphorylation of this serine residue is discussed.