Three approaches to one problem: protein folding in the periplasm, the endoplasmic reticulum, and the intermembrane space.

SIGNIFICANCE: The bacterial periplasm, the endoplasmic reticulum (ER), and the intermembrane space (IMS) of mitochondria contain dedicated machineries for the incorporation of disulfide bonds into polypeptides, which cooperate with chaperones, proteases, and assembly factors during protein biogenesis. RECENT ADVANCES: The mitochondrial disulfide relay was identified only very recently. The current knowledge of the protein folding machinery of the IMS will be described in detail in this review and compared with the "more established" systems of the periplasm and the ER. CRITICAL ISSUES: While the disulfide relays of all three compartments adhere to the same principle, the specific designs and functions of these systems differ considerably. In particular, the cooperation with other folding systems makes the situation in each compartment unique. FUTURE DIRECTIONS: The biochemical properties of the oxidation machineries are relatively well understood. However, it still remains largely unclear as to how the quality control systems of "oxidizing" compartments orchestrate the activities of oxidoreductases, chaperones, proteases, and signaling molecules to ensure protein homeostasis.