Mitochondrial Protein Transport
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Questions and Answers

Which peroxin is responsible for recognizing N-terminal peroxisomal signal sequences?

  • Pex5
  • Pex14
  • Pex3
  • Pex7 (correct)

    • What is unique about the peroxisomal translocator compared to ER translocators?

  • It can transport only small molecules.
  • It can transport only lipids.
  • It can transport only unfolded proteins.
  • It can transport fully folded and oligomeric proteins. (correct)

    • What is the function of mitochondria?

  • Protein synthesis
  • Cell signaling
  • Cell division
  • ATP synthesis (correct)

    • Where are most mitochondrial proteins encoded?

    <p>Nuclear DNA</p> Signup and view all the answers

    What is the role of signal sequences in mitochondrial precursor proteins?

    <p>To direct proteins to their appropriate mitochondrial sub-compartment</p> Signup and view all the answers

    What is the function of the inner mitochondrial membrane?

    <p>Encloses the matrix space and forms cristae</p> Signup and view all the answers

    What is the intermembrane space subdivided into?

    <p>Crista space and intermembrane space</p> Signup and view all the answers

    What type of protein complexes mediate protein movement across or into mitochondrial membranes?

    <p>Multi-subunit protein complexes</p> Signup and view all the answers

    How are new mitochondria produced?

    <p>By the growth of preexisting organelles followed by fission</p> Signup and view all the answers

    Where are protein translocator complexes located?

    <p>In both the inner and outer mitochondrial membranes</p> Signup and view all the answers

    What is a unique feature of the peroxisomal translocator?

    <p>It dynamically adapts in size to the cargo molecule</p> Signup and view all the answers

    What is the function of signal peptidase in mitochondrial protein import?

    <p>To remove signal sequences from imported proteins</p> Signup and view all the answers

    How many different precursor proteins can be directed to the appropriate sub-compartment of mitochondria by protein translocators?

    <p>1500</p> Signup and view all the answers

    What determines which translocator a precursor protein engages and the order in which the signals are used?

    <p>The organization of signals in the precursor protein</p> Signup and view all the answers

    Which complex is involved in the assembly of mitochondrially encoded membrane proteins with nuclear-encoded membrane proteins?

    <p>OXA complex</p> Signup and view all the answers

    What is the main function of the TIM 5 translocator?

    <p>Translocation of proteins across the inner mitochondrial membrane</p> Signup and view all the answers

    Which of the following proteins are synthesized by mitochondrial ribosomes?

    <p>Mitochondrially encoded membrane proteins</p> Signup and view all the answers

    What is the role of chaperones of the hsp70 family in protein import by mitochondria?

    <p>Interaction with mitochondrial precursor proteins</p> Signup and view all the answers

    What is the mechanism of protein import by mitochondria?

    <p>Post-translational</p> Signup and view all the answers

    Which complex is involved in the sorting and assembly of mitochondrial proteins?

    <p>SAM complex</p> Signup and view all the answers

    What is the function of the TOM 5 translocator?

    <p>Translocation of proteins across the outer mitochondrial membrane</p> Signup and view all the answers

    What happens to mitochondrial precursor proteins immediately after they are synthesized?

    <p>They interact with chaperones of the hsp70 family</p> Signup and view all the answers

    What is the primary function of the interacting proteins in mitochondrial protein import?

    <p>To prevent the aggregation of precursor proteins before they engage with the TOM complex</p> Signup and view all the answers

    What determines the fate of a translocating protein in the intermembrane space?

    <p>Sequences within the polypeptide chain</p> Signup and view all the answers

    Which complex can operate independently of the TOM complex?

    <p>TIM complex</p> Signup and view all the answers

    What is the primary source of energy for maintaining the polypeptide in an unfolded state before import?

    <p>Chaperone interactions</p> Signup and view all the answers

    How many discrete sites utilize energy for mitochondrial protein import?

    <p>4</p> Signup and view all the answers

    What is the role of the membrane potential in protein import?

    <p>To pump H+ from the matrix space to the intermembrane space</p> Signup and view all the answers

    What is the source of energy that drives the pumping of H+ from the matrix space to the intermembrane space?

    <p>Electron transport processes</p> Signup and view all the answers

    What is the purpose of the initial use of energy in mitochondrial protein import?

    <p>To maintain the polypeptide in an unfolded state</p> Signup and view all the answers

    What drives the translocation of positively charged signal sequences through the TIM complexes?

    <p>Electrophoresis from the H+ gradient</p> Signup and view all the answers

    What is the role of mitochondrial hsp70 in protein import?

    <p>It undergoes an ATP-dependent conformational change to exert a pulling force on the imported protein</p> Signup and view all the answers

    What is the energy source for the import of certain intermembrane space proteins?

    <p>Energy from the redox potential between the cytosol and mitochondria</p> Signup and view all the answers

    What is the function of Mia40 in protein import?

    <p>It prevents the backsliding of imported proteins through the TOM complex into the cytosol</p> Signup and view all the answers

    What is the result of the interaction between Mia40 and imported proteins in the intermembrane space?

    <p>The imported proteins are released in an oxidized form containing intrachain disulfide bonds</p> Signup and view all the answers

    What is the role of the H+ gradient in ATP synthesis?

    <p>It powers most of the cell's ATP synthesis by ATP synthase complexes</p> Signup and view all the answers

    What is the function of mitochondrial hsp60 in protein import?

    <p>It is a chaperone protein that assists in the folding of imported proteins</p> Signup and view all the answers

    What is the fate of Mia40 after releasing an imported protein in the intermembrane space?

    <p>It becomes reduced and is then reoxidized by passing electrons to the electron transport chain</p> Signup and view all the answers

    Study Notes

    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.

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    Molecular Biology of the Cell Chapter 12 Intracellular Organization and Protein Sorting (Part 2) PDF

    Description

    Learn about the transport of proteins into mitochondria, including signal sequences, protein translocators, and the process of protein import.

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