Protein Translocation and TIM23 Complex
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Questions and Answers

What is the role of the stop-transfer sequence in protein translocation across the mitochondrial inner membrane?

  • It directs the protein to the mitochondrial outer membrane.
  • It initiates protein translocation by binding to the TIM23 complex.
  • It acts as a signal for the protein to be released laterally into the inner membrane. (correct)
  • It triggers the folding of the protein within the mitochondrial matrix.

Which of the following is NOT a component of the TIM23 complex?

  • Mgr2
  • Tim23
  • Tim17
  • Tom40 (correct)

Which of the following statements is TRUE regarding the TOM-TIM23 supercomplex?

  • The TIM23 complex is responsible for the import of proteins into the mitochondrial intermembrane space.
  • The TOM complex is a voltage sensor.
  • The TOM-TIM23 supercomplex forms a continuous translocation pathway across both mitochondrial membranes. (correct)
  • The TOM complex is involved in the release of proteins into the mitochondrial matrix.

What is the role of the aromatic ring in the TIM23 channel?

<p>It prevents the leakage of ions and other small molecules from the mitochondrial matrix. (A)</p> Signup and view all the answers

Which of the following provides a positively charged environment for the polypeptide substrates during their passage through the TOM-TIM23 supercomplex?

<p>The Mgr2 and Tim17 subunits of the TIM23 complex (C)</p> Signup and view all the answers

The TOM-TIM23 supercomplex is a dynamic structure that can change its conformation. Which of the following is a potential mechanism for regulating the activity of the TOM-TIM23 supercomplex?

<p>All of the above (D)</p> Signup and view all the answers

What is the role of mHsp60 in protein translocation?

<p>mHsp60 assists in the folding of the protein after it has been translocated into the matrix. (C)</p> Signup and view all the answers

What is the difference between the N-terminal signal sequence and the stop-transfer sequence?

<p>The N-terminal signal sequence initiates protein translocation, while the stop-transfer sequence halts translocation and releases the protein laterally into the inner membrane. (A)</p> Signup and view all the answers

Which of the following statements is TRUE regarding the transport of lipids across membranes in bacteria?

<p>Lipid transport requires energy input at some or all stages due to the unfavorable nature of lipids in water. (B)</p> Signup and view all the answers

What is the main challenge faced by bacteria with regards to lipid transport in their cell envelope?

<p>The presence of a unique outer membrane organization in Gram-negative bacteria poses a logistical problem for lipid transport. (C)</p> Signup and view all the answers

What is the primary function of SRP in the context of lipid transport?

<p>SRP directs the integration of certain proteins into the chloroplast thylakoid membrane. (A)</p> Signup and view all the answers

Which of the following is NOT a key mechanism for lipid transport across membranes?

<p>Passive diffusion across the lipid bilayer. (B)</p> Signup and view all the answers

Which of the following statements is TRUE regarding the biogenesis of β-barrel proteins in Gram-negative bacteria?

<p>They require specialized machinery for their biogenesis, distinct from the machinery for other membrane proteins. (A)</p> Signup and view all the answers

Which of the following proteins is involved in the integration of light-harvesting chlorophyll a, b binding proteins into the chloroplast thylakoid membrane?

<p>Alb3 (C)</p> Signup and view all the answers

What is the primary function of Lipopolysaccharides (LPS) in the bacterial cell envelope?

<p>To protect the bacteria against environmental stresses. (B)</p> Signup and view all the answers

Which of the following organelles exhibits structural homology with the bacterial cell envelope?

<p>Mitochondria (B)</p> Signup and view all the answers

What structural component is essential for BamA's function in the biogenesis of β-barrel membrane proteins?

<p>C-terminal β-barrel domain (C)</p> Signup and view all the answers

Which feature of the POTRA domains contributes to their function in the recognition of β-strands?

<p>Conformational variability (A)</p> Signup and view all the answers

How do the β-strands interact with BamA during the protein folding process?

<p>By β-augmentation (B)</p> Signup and view all the answers

Which characteristic of the BamA architecture enhances its functionality?

<p>16-strand β-barrel structure (B)</p> Signup and view all the answers

What is the significance of the lateral opening and exit pore formation in BamA?

<p>Essential for OMP transfer (C)</p> Signup and view all the answers

The flexibility in the POTRA domains is largely attributed to which feature?

<p>Hinge regions like PD2/3 (C)</p> Signup and view all the answers

Which other component is involved alongside BamA in the process of OMP biogenesis?

<p>SurA (C)</p> Signup and view all the answers

In the context of BamA, what role do the POTRA motifs play?

<p>Recognition of unfolded OMPs (A)</p> Signup and view all the answers

What is the main role of the SAM (Sorting and Assembly Machinery) in mitochondria?

<p>Assists in the folding and integration of β-barrel proteins into the outer membrane. (C)</p> Signup and view all the answers

Which protein structure is primarily associated with the function of porins in the outer mitochondrial membrane?

<p>β-barrel structure. (C)</p> Signup and view all the answers

What prevents the aggregation of β-barrel proteins during their transport in mitochondria?

<p>Chaperone proteins. (C)</p> Signup and view all the answers

What is the significance of the β-signal in the C-terminal β-strand?

<p>It is crucial for the assembly of β-barrel membrane proteins. (B)</p> Signup and view all the answers

Which complex forms when SAM interacts with TOM?

<p>SAM-TOM supercomplex. (A)</p> Signup and view all the answers

How is the system involving SAM and TOM in mitochondria similar to that found in Gram-negative bacteria?

<p>Both utilize similar mechanisms for β-barrel protein assembly. (C)</p> Signup and view all the answers

Which domain is involved in the assembly of β-barrel proteins using the SAM system?

<p>POTRA domain. (C)</p> Signup and view all the answers

What defines the TOM (Translocase of the Outer Membrane) in the context of mitochondrial protein insertion?

<p>It is responsible for integrating β-barrel proteins into the outer membrane. (A)</p> Signup and view all the answers

Which complex is primarily responsible for the import of most mitochondrial proteins?

<p>Translocase of the Outer Membrane (TOM) (C)</p> Signup and view all the answers

What is the role of mitochondrial processing peptidase (MPP) in mitochondrial protein import?

<p>To cleave the presequences from imported proteins (D)</p> Signup and view all the answers

Which pathway is associated with the import of cysteine-rich intermembrane space proteins?

<p>Mitochondrial intermembrane space machinery (MIA) (B)</p> Signup and view all the answers

What is a major difference between mitochondrial protein transport and bacterial β-barrel assembly?

<p>The direction of transport differs (D)</p> Signup and view all the answers

Which machinery is responsible for the assembly of mitochondrial-encoded inner membrane proteins?

<p>Export and oxidase assembly machinery (OXA) (A)</p> Signup and view all the answers

What is a common characteristic of the protein transport systems in chloroplasts compared to those in mitochondria and bacteria?

<p>They are homologous to bacterial systems (C)</p> Signup and view all the answers

Which component is involved in the translocation of mitochondrial matrix proteins?

<p>Δψ-dependent presequence translocase of the inner membrane (TIM23) (A)</p> Signup and view all the answers

How are β-barrel precursor proteins synthesized in the mitochondria?

<p>In the cytosol and then imported via TOM (D)</p> Signup and view all the answers

Which of the following describes the main function of ABC transporters?

<p>Transport lipids and other substances across the cell membrane (B)</p> Signup and view all the answers

What is the structural representation of the MlaD protein?

<p>A hexameric structure resembling a stack of rings (C)</p> Signup and view all the answers

What is the role of MlaC in bacterial lipid transport?

<p>Serves as a lipid shuttle to maintain membrane integrity (C)</p> Signup and view all the answers

Which of the following correctly identifies a component of the MlaDEFB transporter?

<p>MlaF2 (D)</p> Signup and view all the answers

What characteristic is associated with the MlaD/MCE family transporters?

<p>Maintain lipid asymmetry across bacterial membranes (B)</p> Signup and view all the answers

What distinguishes spontaneous and energy-dependent transport mechanisms in bacterial lipid transport?

<p>Energy-dependent transport requires ATP hydrolysis to function effectively (D)</p> Signup and view all the answers

Which of the following proteins is a flippase involved in lipid transport?

<p>MsbA (A)</p> Signup and view all the answers

What is the significance of LetB in bacterial lipid transport?

<p>Facilitates lipid entry into the cytosol (C)</p> Signup and view all the answers

Flashcards

Oxa1

A protein essential for assembling mitochondrial inner membrane proteins like cytochrome c oxidase and ATP synthase.

Alb3

A mediator that integrates light-harvesting proteins into chloroplast thylakoid membranes.

Gram-negative bacteria

Bacteria that have a unique outer membrane structure with two membranes and special proteins.

Lipopolysaccharides (LPS)

Large molecules found in the outer membrane of Gram-negative bacteria, critical for maintaining structural integrity.

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Bacterial lipid transport

The processes involved in moving lipids across membranes, often requiring energy due to unfriendly water interaction.

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Structural homology

The similarity in structure among molecular machines across different organisms, such as bacteria and organelles.

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Energy input in lipid transport

The necessary energy required to transfer lipids across a water barrier, making it energetically unfavorable.

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Bacterial cell envelope

The outer protective layer of bacteria, consisting of unique structures that differentiate Gram-positive from Gram-negative types.

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ABC Transporter

ATP-Binding Cassette transporter involved in transport processes across membranes.

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MsbA

A specific ABC transporter that functions as a flippase, moving lipid molecules across membranes.

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MlaC

A protein that plays a role in lipid transport, helping to maintain lipid asymmetry in bacterial membranes.

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MlaD

A member of the MCE family that acts as a lipid shuttle in bacterial cells.

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YebT/LetB

Proteins involved in bacterial lipid transport, known for their roles in transport mechanisms.

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MlaDEFB

A transporter complex representing the ABC family, involved in lipid transport across membranes.

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Anterograde vs Retrograde Transport

Anterograde transport moves substances towards the cell exterior, while retrograde transport moves them back towards the cell interior.

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Periplasm

The space between the inner and outer membranes in Gram-negative bacteria.

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BamA

A key protein involved in the assembly of β-barrel outer membrane proteins (OMPs).

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POTRA motifs

Five domains in BamA that recognize and bind to β-strands of OMPs.

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β-barrel

A protein structure formed by β-strands that create a cylindrical shape in membranes.

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OMP

Outer membrane proteins that are crucial for the function of bacterial membranes.

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DegP

A periplasmic chaperone that assists in protein folding and degradation.

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Lateral opening

The mechanism through which BamA facilitates the entry of OMPs into the membrane.

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Pymol

A molecular visualization program to view structures like BAM in 3D.

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Hsp70

A protein that keeps polypeptides unfolded in the cytosol for TOM passage.

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mHsp70

Mitochondrial Hsp70 that uses ATP to help transport proteins into the matrix.

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TIM23

A channel that facilitates the transport of proteins into the inner mitochondrial membrane.

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Signal Sequence

A sequence that targets proteins for transport to the inner mitochondrial membrane.

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Stop-transfer sequence

A sequence that halts protein transport, allowing lateral release into the inner membrane.

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Tom40

A subunit of the TOM complex that helps form the translocation pathway for proteins.

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Mgr2

A TIM23 subunit that shields polypeptides from lipids during transport.

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Voltage sensor in TIM23

A component of TIM23 that detects voltage changes for transport regulation.

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Porins

Common β-barrel proteins found in the outer mitochondrial membrane.

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TOM Complex

Translocase of the Outer Membrane, it helps transport proteins into mitochondria.

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SAM Machinery

Sorting and Assembly Machinery assists in inserting β-barrel proteins in the outer membrane.

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Chaperones

Proteins that prevent aggregation and assist in folding other proteins correctly.

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β-Barrel Structure

A specific protein conformation that forms pores in membranes, crucial for porins.

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POTRA Domain

A domain involved in the assembly of β-barrel proteins by the SAM complex.

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Endosymbiosis Evidence

Indicates evolutionary links between mitochondria and Gram-negative bacteria.

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TOM–SAM Supercomplex

A structure formed by the interaction of TOM and SAM to facilitate protein insertion.

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Mitochondrial Protein Import

Over 99% of mitochondrial proteins are nuclear-coded and imported via the TOM complex.

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Translocase of the Outer Membrane (TOM)

A protein complex that facilitates the import of mitochondrial proteins from the cytosol.

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MIA Machinery

Machinery for importing and assembling cysteine-rich intermembrane space proteins in mitochondria.

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Presequence Translocase-Associated Motor (PAM)

ATP-driven system that helps in translocating mitochondrial proteins into the matrix.

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Mitochondrial Processing Peptidase (MPP)

Enzyme that cleaves presequences from imported mitochondrial proteins in the matrix.

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OXA Machinery

Involved in the assembly of mitochondrial inner membrane proteins that are encoded by mitochondria.

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Homology in Protein Transport

The similarity in protein transport systems across mitochondria, chloroplasts, and bacteria.

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Study Notes

Membrane Transport in Bacteria and Organelles

  • Gram-negative bacteria have a unique outer membrane with complex processes for phospholipid transport, LPS (lipopolysaccharide) systems (Wzz, MsbA, and Lpt), beta-barrel protein systems (BAM), and capsules.
  • Protein transport into mitochondria involves TOM (translocase of the outer membrane) and TIM (translocase of the inner membrane) complexes. SAM (sorting and assembly machinery) also plays a role
  • Protein transport into plastids involves the Tic/Toc system.
  • Post-translational protein translocation: proteins are folded before transport, examples include the Tat pathway
  • Co-translational protein translocation: proteins are unfolded while being transported, examples include SecY/Sec61 associated with the ribosome
  • Outer membrane protein (OMP) biogenesis is a key process in Gram-negative bacteria, where all OMPs are beta-barrel proteins.

Bacterial Lipid Transport

  • There are several mechanisms for lipid transport across membranes in bacteria that use energy.
  • Transport machineries include the Lpt, Lol, and OmpC-Mla pathways. These mechanisms differ in whether lipid transport is co- or post-translational.

Bacterial Lipid Transport Machinery

  • The Lpt machine is a sophisticated and important lipid transport system.
  • The Lol pathway is a conserved pathway for transporting lipids.
  • The OmpC-Mla pathway is used to transport lipids between membranes.
  • Energy is required in all stages of lipid transport across membranes.

YidC/Oxa1/Alb3 Family

  • A new insertase family called YidC/Oxa1/Alb3 has been identified and is present in all kingdoms of life.
  • In bacteria, YidC acts as a chaperone and facilitates the proper folding of membrane proteins.

Protein Transport into Organelles (Mitochondria and Chloroplasts)

  • Proteins for mitochondria and chloroplasts are largely coded in the nucleus.
  • Proteins need to cross at least one membrane (often two) to reach their target site.
  • Most mitochondrial proteins are transported post-translationally.
  • Precursor proteins have an N-terminal signal sequence (amphiphilic) to be recognised by import receptors (e.g., TOM, TIM).

TOM and TIM Complexes

  • TOM complex is crucial for transporting proteins into/through the outer mitochondrial membrane.
  • TIM complexes (TIM22 and TIM23) are involved in transporting proteins into the inner membrane. OXA complex is involved in membrane protein insertion from inside out (matrix to IM membrane).
  • The components of TOM (Tom20, Tom22, Tom70, Tom40 etc.) and TIM are central to protein transport.
  • Differences in pathways exist for different types of proteins (e.g., β-barrel proteins use a distinct system)
  • Mitochondria and chloroplasts have similar β-barrel protein-import mechanisms, mirroring those in gram-negative bacteria.

Mitochondrial RNA Import

  • RNA import into mitochondria varies depending on the organism, with some suggesting differing mechanisms, such as specialized import factors.
  • There is evidence from some research that RNA transport into mitochondria is different in eukaryotes and prokaryotes

OMP Biogenesis

  • Outer membrane proteins (OMPs) enter the periplasm through the Sec pathway.
  • OMPs are bound and stabilized by chaperones (SurA, Skp, and DegP) in the periplasm before reaching the beta barrel assembly machinery.
  • BAM machinery inserts OMPs into the membrane.
  • E. coli: BamA and four lipoproteins (BamB, C, D, and E) are essential for this process.
  • BamA's role may involve creating a lateral opening in its beta barrel to facilitate substrate folding.
  • The process is highly conserved but with some variability in bacteria.

Bacterial Lipid Transport

  • Bacterial systems for transporting lipids are complex and include multiple components.

LPS Biogenesis

  • O-antigen subunits are translocated across the inner membrane.
  • Core-Lipid A is flipped by a core lipid A-flippase.
  • The Wzx and Wzy systems are crucial for O-antigen polymerization.

LPS Intermembrane Transport

  • LPS traverses membranes using the unique system of jellyroll domains within Lpt proteins, traversing the periplasm.

Chloroplast Transport

  • Import pathways in chloroplasts are broadly similar but with some key differences to mitochondrial processes.
  • TOCs (outer membrane translocators) and TICs (inner membrane translocators): essential components in chloroplast protein import.
  • The complex pathways include specific signal sequences for different proteins.

Additional Note

  • The study of membrane transport in bacteria and organelles has revealed sophisticated mechanisms for protein and lipid transport. The presence of several pathways and machinery highlights how important these mechanisms are for maintaining cellular activities.

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Description

This quiz explores key concepts related to protein translocation across the mitochondrial inner membrane, focusing on the TOM-TIM23 supercomplex and its components. It covers the roles of specific sequences and proteins involved in this intricate process. Test your understanding and knowledge about the mechanisms of protein transport and lipid dynamics in bacterial membranes.

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