Mitochondrial and Chloroplast Protein Import: Structures, Pathways, and Complexes

Where are 99% of mitochondrial matrix proteins produced?

On cytoplasmic ribosomes.

What is the structure of the N-terminal signal sequence in mitochondrial proteins?

An amphiphilic alpha helix.

What recognizes the N-terminal signal sequence during protein import into mitochondria?

A membrane receptor.

How are mitochondrial proteins imported into the mitochondria?

Post-translationally.

What happens to the signal sequence once the protein is in the mitochondrial matrix?

It is cleaved.

What is the function of the TOM complex?

Transfers proteins across the outer membrane of mitochondria.

What is the role of the TIM complex?

Transfers proteins across the inner membrane of mitochondria.

What is the process by which proteins unfold to enter mitochondria?

The import receptor of the TOM complex binds the signal sequence of the precursor protein.

What assists in the translocation of proteins through the TOM/TIM complexes?

Hsp70 chaperones.

What drives protein translocation into mitochondria?

ATP hydrolysis and proton gradients.

What are the two pathways for protein translocation into the inner mitochondrial membrane?

1. Protein stops at TIM23 complex and remains in the inner membrane. 2. Protein is delivered into the matrix and then inserted into the inner membrane by the OXA complex.

How are outer membrane proteins translocated?

They are transported unfolded into the intermembrane space by the TOM complex and inserted into the outer membrane by the SAM complex.

What complexes are required for mitochondrial protein translocation?

TOM complex, TIM(22/23) complex, SAM complex, and OXA complex.

What is the chloroplast equivalent of the TOM complex?

TOC (translocase of outer membrane of the chloroplast) complex.

What is the chloroplast equivalent of the TIM complex?

TIC (translocase of inner membrane of the chloroplast) complex.

What directs proteins inside the chloroplast?

A signal peptide at the N-terminus with no well-defined sequence.

What energy sources drive protein import into chloroplasts?

ATP and GTP.

What internal mechanism assists in protein translocation within the TIC complex?

Internal motors.