Immune signalling by redox-based, post-translational protein modifications: Symposium keynote

Post-translational protein modifications add a tremendous amount of complexity to cellular proteomes. The large variety of post-translational modifications and their concurrent appearance in proteins dramatically increase the proteome size from mere thousands to the order of millions of possible protein forms. Most modifications are reversible due to the action of specific enzymes, allowing them to be used as signalling switches that control protein function. While this process is understood relatively well for many modifications, the way in which redox-based modifications are directly controlled remains largely unknown.
Cellular redox changes mediate signalling events and responses to the environment in eukaryotic cells. Responses to environmental stress are frequently associated with bursts of reactive oxygen and nitrogen species that can modify cysteine thiols of signalling proteins. Thiol reactivity towards these oxidizing agents leads to formation of, amongst others, disulfides and S-nitrosothiols (SNO), which may alter the function, localization, or activity of signalling proteins that harbour them. Because these modifications occur spontaneously, we questioned how redox-based modifications are employed by the cell as specific, reversible signalling cues. Here we show that the plant immune system utilizes novel and well-established members of the redoxin family, a class of oxidoreductases, to directly engineer the thiol redox states of immune signalling proteins. We will discuss how several redoxins, some unexplored while others well-known, exhibit novel molecular modes of action and may introduce previously unrecognized specificity into redox-based signalling networks that lead to immune gene expression.