Tic22 Is Targeted to the Intermembrane Space of Chloroplasts by a Novel Pathway

Tic22 previously was identified as a component of the general import machinery that functions in the import of nuclear-encoded proteins into the chloroplast. Tic22 is peripherally associated with the outer face of the inner chloroplast envelope membrane, making it the first known resident of the intermembrane space of the envelope. We have investigated the import of Tic22 into isolated chloroplasts to define the requirements for targeting of proteins to the intermembrane space. Tic22 is nuclear-endoded and synthesized as a preprotein with a 50-amino acid N-terminal presequence. The analysis of deletion mutants and chimerical proteins indicates that the precursor of Tic22 (preTic22) presequence is necessary and sufficient for targeting to the intermembrane space. Import of preTic22 was stimulated by ATP and required the presence of protease-sensitive components on the chloroplast surface. PreTic22 import was not competed by an excess of an authentic stromal preprotein, indicating that targeting to the intermembrane space does not involve the general import pathway utilized by stromal preproteins. On the basis of these observations, we conclude that preTic22 is targeted to the intermembrane space of chloroplasts by a novel import pathway that is distinct from known pathways that target proteins to other chloroplast subcompartments.