Switching the sorting mode of membrane proteins from co-translational ER targeting to post-translational mitochondrial import

Hydrophobic membrane proteins are co-translationally targeted to the ER membrane, mediated by hydrophobic signal sequence. Mitochondrial membrane proteins escape this mechanism despite of the hydrophobic character. We examined sorting of membrane proteins into the mitochondria, using mitochondrial ABC transporter isoform (ABC-me). In the absence of 135-residue N-terminal hydrophilic segment (N135), the membrane domain was integrated into the ER membrane in COS7 cells. Other sequences that were sufficient to import soluble protein into mitochondria could not import the membrane domain. N135 imports other membrane proteins into mitochondria. N135 prevents co-translational targeting of the membrane domain to ER and in turn achieves post-translational import into mitochondria. In a cell-free system, N135 suppresses targeting to the ER membranes, although it does not affect recognition of hydrophobic segments by signal recognition particle. We conclude that the N135 segment blocks the ER targeting of membrane proteins even in the absence of mitochondria and switches the sorting mode from co-translational ER integration to post-translational mitochondrial import.