Figure 1.

Only the Cys-reduced small Tim precursor proteins are import-competent. The small Tim proteins have to be kept in their reduced conformation while in the cytosol. Neither the fully nor the partially oxidized proteins can be imported into mitochondria. Oxidation could be brought about by GSSG, oxygen or reactive oxygen species (ROS). Partially oxidized proteins can be reduced by the cytosolic Thioredoxin system. Additionally, the reduced proteins could be stabilized by zinc binding, although it is uncertain whether the zinc-bound form can be imported directly.