Question

compare and contrast how proteins are localized to the matrix of mitochondria, the outer membrane and the inner membrane.

Answer 1

ANS : Mitochondria contain their own genome, most mitochondrial proteins are encoded by nuclear genes, necessitating a mechanism to target and import those proteins into mitochondria.  Mitochondrial proteins contain a signal sequence that targets them to mitochondria. Unlike ER proteins, mitochondrial proteins are imported post-translationally. Protein import into mitochondria is similar to import into the ER but is complicated by the presence of two membrane around mitochondria. Mitochondrial proteins can reside in the outer membrane, inner membrane, intermembrane space, or matrix .

The signal sequence that targets proteins to the matrix usually resides at the N-terminus. The signal sequence is recognized by proteins in the TOM complex. The TOM complex passes the proteins into the inner membrane space where the TIM complex in the inner membrane passes the protein into the matrix. The TOM and TIM complex often work together to translocate a protein across both membranes. Translocation across mitochondrial membranes is energy dependent. Chaperones within the matrix help “pull” the protein across the inner membrane and require ATP hydrolysis to function. The proteins fold inside the matrix.

Proteins targeted to the inner membrane use a similar mechanism as matrix proteins but contain a stop transfer sequence recognized by the TIM complex. Proteins targeted to the outer membrane are translocated across the outer membrane into the intermembrane space and then imported into the outer membrane by the SAM translocator. Proteins targeted for the intermembrane space are partially inserted into the inner membrane and then cleaved by a protease and released into the inner membrane space.