TIC20 AND TIC22 ARE NEW COMPONENTS OF THE PROTEIN IMPORT APPARATUS ATTHE CHLOROPLAST INNER ENVELOPE MEMBRANE

Two components of the chloroplast envelope, Tic20 and Tic22, were prev iously identified as candidates for components of the general protein import machinery by their ability to covalently cross-link to nuclear- encoded preproteins trapped at an intermediate stage in import across the envelope (Kouranov, A., and D.J. Schnell. 1997. J. Cell Biol. 139: 1677-1685). We have determined the primary structures of Tic20 and Tic 22 and investigated their localization and association within the chlo roplast envelope. Tic20 is a 20-kD integral membrane component of the inner envelope membrane. In contrast, Tic22 is a 22-kD protein that is located in the intermembrane space between the outer and inner envelo pe membranes and is peripherally associated with the outer face of the inner membrane. Tic20, Tic22, and a third inner membrane import compo nent, Tic110, associate with import components of the outer envelope m embrane. Preprotein import intermediates quantitatively associate with this outer/inner membrane supercomplex, providing evidence that the c omplex corresponds to envelope contact sites that mediate direct trans port of preproteins from the cytoplasm to the stromal compartment. On the basis of these results, we propose that Tic20 and Tic22 are core c omponents of the protein translocon of the inner envelope membrane of chloroplasts.