Abstract:
Reporter gene transactivation by human p53 is inhibited in budding yeast lacking the TRR1 gene
encoding thioredoxin reductase. To investigate the role of thioredoxin in controlling p53 activity,
the level of reporter gene transactivation by p53 was determined in yeast lacking the TRX1 and
TRX2 genes encoding cytosolic thioredoxin. Surprisingly, p53 activity was unimpaired in yeast
lacking thioredoxin. Subsequent analyses showed that thioredoxin deletion suppressed the inhibitory
effect of thioredoxin reductase deletion, suggesting that accumulation of oxidized thioredoxin in
mutant yeast was necessary for p53 inhibition. Purified human thioredoxin and p53 interacted in
vitro (Kd = 0.9 µM thioredoxin). To test the idea that dithio-disulfide exchange reactions between
p53 and thioredoxin were responsible for p53 inhibition in mutant yeast, each p53 cysteine was
changed to serine and the effect of the substitution on p53 activity in TRR1 and Δtrr1 yeast was
determined. Substitutions at Zn-coordinating cysteines C176, C238 or C242 resulted in p53
inactivation. Unexpectedly, substitution at cysteine C275 also inactivated p53, which was the first
evidence for a non-zinc-coordinating cysteine being essential for p53 function. Cysteine substitutions
at six positions (C124, C135, C141, C182, C229 and C277) neither inactivated p53 nor relieved the
requirement for thioredoxin reductase. Furthermore, no tested combination of these six cysteine
substitutions relieved thioredoxin reductase dependence. The results suggested that p53 dependence
on thioredoxin reductase either was indirect, perhaps mediated by an upstream activator of p53, or
was due to oxidation of one or more of the four essential cysteines.
