The expanded specificity and physiological role of a widespread N-degron recognin.

All cells use proteases to maintain protein homeostasis. The proteolytic systems known as the N-degron pathways recognize signals at the N terminus of proteins and bring about the degradation of these proteins. The ClpS protein enforces the N-degron pathway in bacteria and bacteria-derived organelles by targeting proteins harboring leucine, phenylalanine, tryptophan, or tyrosine at the N terminus for degradation by the protease ClpAP. We now report that ClpS binds, and ClpSAP degrades, proteins still harboring the N-terminal methionine. We determine that ClpS recognizes a type of degron in intact proteins based on the identity of the fourth amino acid from the N terminus, showing a strong preference for large hydrophobic amino acids. We uncover natural ClpS substrates in the bacterium Salmonella enterica, including SpoT, the essential synthase/hydrolase of the alarmone (p)ppGpp. Our findings expand both the specificity and physiological role of the widespread N-degron recognin ClpS.