Inactivation of Saccharomyces cerevisiae Sulfate Transporter Sul2p: Use It and Lose It

Abstract

Saccharomyces cerevisiae SO4= transport is regulated over a wide dynamic range. Sulfur starvation causes ∼10,000-fold increase in the 35SO4= influx mediated by transporters Sul1p and Sul2p; >80% of the influx is via Sul2p. Adding methionine to S-starved cells causes a 50-fold decline (t1/2 ∼5 min) in SUL1 and SUL2 mRNA but a slower decline (t1/2 ∼1 h) in transport. In contrast, SO4= addition does not affect mRNA but causes a rapid (t1/2 = 2–4 min) decrease in transport. In met3Δ cells (unable to metabolize SO4=), addition of SO4= to S-starved cells causes inactivation of 35SO4= influx over times in which cellular SO4= contents are nearly constant. The relationship between cellular SO4= and transport inactivation shows that cellular SO4= is not the signal for Sul2p inactivation. Instead, the transport inactivation rate has the same dependence on extracellular SO4= as 35SO4= influx, indicating that Sul2p exhibits use-dependent inactivation; the transport process itself increases the probability of Sul2p inactivation and degradation. In addition, there is a transient efflux of SO4= shortly after adding >0.02 mM SO4= to S-starved met3Δ cells. This transient efflux provides further protection against excessive SO4= influx and may represent an alternate transport mode of Sul2p.