Protein thiol oxidative stress
Protein thiols are targets of oxidative stress. In all organisms, a variety of reductive pathways ensure that cytosolic thiol groups are maintained in their reduced state and prevent the formation of stable disulphide bonds (Figure 3). Oxidative stress leads to the formation of unwanted disulphide bonds in the cytoplasm, a process termed disulphide stress. Upon return to non-oxidative conditions, cellular reductases like thioredoxin reduce the cysteine modifications and restore the original protein activity.
Two thiol side-chains (-SH) can be oxidized reversibly to a disulphide (-S-S-). Thioredoxin catalyzes the reduction of disulphide bonds.
In Bacillus subtilis the global regulator, Spx, has a key role in helping the organism to cope with disulphide stress. Spx controls a regulon of more than 100 genes that help to re-establish cytoplasmic thiol disulphide redox poise and to repair damage caused by the stress (Figure 4). We explore the complex transcriptional and post-translation control that regulates the cellular activity of Spx.
Controlled proteolysis is a common strategy to remove regulatory proteins when they are not needed. The ATP-powered AAA+ protease, ClpXP, degrades the thiol oxidative transcription regulator, Spx, under non-stress conditions. The novel protein YjbH facilitates ClpXP recognition of the Spx C-terminal, possibly by interacting with the C-terminal part of Spx and thereby making it more accessible, resulting in rapid degradation of Spx by the ClpXP protease. Under disulphide stress, YjbH no longer enhances recognition, leading to a decreased Spx degradation rate. Spx acts as a positive regulator of the yjbIH operon, creating a regulatory loop.