Mitochondrial Protein Transport

Mitochondrial Protein Transport

• Mitochondrial protein transport depends on signal sequences and protein translocators.
• One or more signal sequences direct all mitochondrial precursor proteins to their appropriate sub-compartment.
• Many proteins entering the matrix space contain a signal sequence at their N-terminus that is rapidly removed after import.
• Imported proteins, including outer membrane and inner membrane proteins, have internal signal sequences that are not removed.

Protein Translocators in Mitochondrial Membranes

• Multi-subunit protein complexes, called protein translocators, mediate protein movement across or into mitochondrial membranes.
• These complexes are located in both the inner and outer mitochondrial membranes.
• They recognize particular types of signals and direct ~1500 different precursor proteins from the cytosol to the appropriate sub-compartment.
• The organization of signals in a precursor protein controls which translocator(s) the protein engages and the order in which the signals are used to reach the final destination.

Mitochondrial Structure and Function

• Mitochondria is a double membrane-enclosed organelle specialized in ATP synthesis, using energy derived from electron transport and oxidative phosphorylation.
• Although it contains its own DNA, ribosomes, and other components required for protein synthesis, almost all of its proteins are encoded in the cell nucleus and imported from the cytosol.
• Each imported protein must reach the particular organelle sub-compartment in which it functions.
• The sub-compartments of mitochondria are formed by the two concentric mitochondrial membranes: inner, outer, and intermembrane space.

Mitochondrial Protein Import

• Mitochondrial precursor proteins do not immediately fold into their native structures after synthesis; instead, some interact with chaperones of the hsp70 family, whereas others are dedicated to mitochondrial precursor proteins and bind directly to their signal sequences.
• The protein translocators in the mitochondrial outer membrane do not bind to ribosomes, and most mitochondrial proteins are imported by a post-translational mechanism.
• Once the translocating protein protrudes into the intermembrane space, sequences within the polypeptide chain determine what happens next.
• Proteins destined for the matrix or inner membrane engage one of the TIM complexes and are either translocated across or inserted into the inner membrane.

Energy Requirements for Protein Import

• Mitochondrial protein import utilizes three different sources of energy at four discrete sites: ATP, membrane potential, and redox potential.
• Energy from ATP hydrolysis is used to maintain the polypeptide in an unfolded state prior to import.
• The membrane potential (electrical component of the electrochemical H+ gradient across the inner membrane) drives the translocation of positively charged signal sequences through the TIM complexes by electrophoresis.
• The energy from the electrochemical H+ gradient also powers most of the cell's ATP synthesis by ATP synthase complexes in the inner mitochondrial membrane.
• Certain inter-membrane space proteins use the difference in redox potential between the cytosol and mitochondria as a source of energy for import.