Mitochondrial Protein Import: Translocons and Mechanisms

Mitochondrial Protein Import: Translocons and Mechanisms

Origin of Mitochondria and Gene Migration

• Mitochondria originated from an endosymbiosis event.

• Initially, the organism's genome would have been self-sufficient.

• Over time, most genes encoding mitochondrial proteins have migrated to and are now encoded in the nucleus of the host cell.

Overview of Mitochondrial Translocons

• Mitochondria contain various translocons (also called translocases), which are protein complexes responsible for protein import.

• These translocons are located in both the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM).

• Focus for this course: Tom (Translocase of the Outer Membrane) and Tim (Translocase of the Inner Membrane).

  • Tom: Essential for nearly all nuclear-encoded proteins to begin their journey into the mitochondria. It recognizes the signal sequence and feeds the protein into the inner membrane space.

  • Tim: Transports proteins from the intermembrane space into the matrix or the inner membrane. We will specifically focus on Tim23, which transports proteins into the matrix and the inner membrane.

• Other Translocons (Briefly mentioned/Contextual):

  • Sam (Sorting and Assembly Machinery): Involved in the insertion of beta-barrel proteins into the OMM.

  • Tim22: Transports some other transmembrane proteins.

  • OXA (Oxidative phosphorylation pathway translocase): Targets proteins to the inner membrane space.

  • MIM (Mitochondrial Inner Membrane translocase): A more recently characterized complex responsible for inserting alpha-helical proteins into the OMM. It is not extensively covered in Alberts's $7^{th}$ edition.

Detailed Mechanism of Tom and Tim Mediated Protein Import

$1^{st}$ Step: Protein Synthesis and Chaperone Escort in the Cytosol

• Proteins destined for mitochondria are synthesized on ribosomes in the cytosol.

• These proteins must remain unfolded (linearized form) to pass through the narrow translocon pores, which are much smaller than nuclear pore complexes.

• Hsp70 chaperones (and potentially other chaperones) bind to the nascent protein in the cytosol, preventing aggregation and maintaining an unfolded state.

  • Hsp70 chaperones function via a clamp and release mechanism that requires ATP hydrolysis ($ATP ightarrow ADP + P_i$).

$2^{nd}$ Step: Arrival at the Outer Mitochondrial Membrane

• The chaperone-escorted protein arrives at the mitochondrial membrane, often at organelle contact sites, facilitating close proximity.

• Tom complex (a large multiprotein complex) recognizes a specific signal sequence (often shown in red in diagrams) on the protein.

  • This is a specific recognition event by subcomponents of the Tom complex.

• The protein is then fed into the Tom translocon.

$3^{rd}$ Step: Passage Through Tom and Transfer to Tim

• As the protein moves through Tom, the associated chaperones in the cytosol unclamp and release the protein, powered by ATP hydrolysis.

• The inner and outer mitochondrial membranes are in close proximity, allowing for direct transfer from Tom to Tim.

$4^{th}$ Step: Passage Through Tim into the Matrix

• Signal Sequence Characteristics: The signal sequence typically contains:

  • Hydrophobic residues.

  • Positively charged residues.

• Energy Input from Membrane Potential: The positively charged residues in the signal sequence are driven across the inner mitochondrial membrane by the existing membrane potential (electrochemical gradient).

• Energy Input from Chaperone Loading (Molecular Ratchet):

  • Once the protein enters the matrix, Hsp70 chaperones (specifically, a mitochondrial matrix Hsp70, distinct from cytosolic Hsp70) are loaded onto the protein.

  • This chaperone loading is an ATP-mediated process ($ATP ightarrow ADP + P_i$), often described as a molecular ratchet mechanism.

  • Two major models for the molecular ratchet:

    1. Sequential loading and pulling: Chaperones are loaded sequentially, acting to pull the protein through the membrane, preventing backsliding.

    2. Stationary attachment and prevention of backsliding: The chaperone remains more stationary at the translocon, but its attachment prevents the protein from sliding back out of the Tim complex into the intermembrane space.

  • Though the specific mechanics differ, both models achieve unidirectional movement and are described as ratchet mechanisms.

• Signal Sequence Cleavage and Protein Folding:

  • After reaching the matrix, specific enzymes (e.g., Mitochondrial Processing Peptidase, MPP) in the matrix cleave off the N-terminal signal sequence.

  • The protein is then released from chaperones and can fold into its final three-dimensional conformation, often assisted by matrix chaperones (e.g., Hsp60 and Hsp70).

Protein Localization: Matrix vs. Inner Membrane Insertion

• The final destination of a protein (soluble in matrix or embedded in IMM) is determined by its amino acid sequence and interactions with the translocon's anatomy.

• Translocon Anatomy (General):

  • Central pore: Through which the protein passes.

  • Constriction ring: Helps with specificity by recognizing signal sequences.

  • Plug domain: Keeps the translocon closed when no protein is passing.

  • Lateral gate: A gating mechanism that allows hydrophobic protein segments to escape laterally into the membrane.

• Stop-Transfer Sequences:

  • Proteins destined for the inner membrane contain stop-transfer sequences in addition to their N-terminal signal sequence.

  • These sequences are typically long (e.g., $20$ to $30$ amino acids) and hydrophobic enough to function as a transmembrane domain.

  • When the stop-transfer sequence enters the translocon, it interacts with the lateral gate, causing the protein segment to be pushed out laterally and embedded into the inner mitochondrial membrane.

  • The N-terminal signal sequence can still be cleaved, and the part of the protein that passed through Tim folds into the matrix, while the segment with the stop-transfer sequence remains in the membrane.

  • This explains one orientation (N-terminus inward) for a single-pass protein. More complex patterns (multi-pass proteins, different orientations) will be discussed for ER protein import.

Alternative Targeting and Specialized Mechanisms

Localization to the Inner Membrane Space

• Proteins targeted to the inner membrane space (between OMM and IMM) often participate in redox reactions.

• Interaction with Mia (Mitochondrial Intermembrane space Assembly) protein, which participates in redox reactions and interacts with the respiratory chain, leads to the formation of disulfide bonds within the imported protein.

• These disulfide bonds produce structural changes that prevent the protein from backsliding out of the intermembrane space, effectively trapping it there.

Insertion of Beta-Barrel Proteins (Sam Complex)

• The Sam complex is specialized for inserting beta-barrel proteins into the outer mitochondrial membrane.

• This mechanism is strikingly similar to how Gram-negative bacteria insert beta-barrels into their outer membranes, reflecting the endosymbiotic origin of mitochondria.

  • In both cases, proteins are translated in the cytosol, exported to a periplasmic/intermembrane-like space, and then inserted via a translocon.

Phospholipid Replenishment

• Mitochondria obtain phospholipids from the endoplasmic reticulum (ER) via carrier proteins that transport phospholipids between the two organelles.

Mitochondrial Deficiencies and Multiprotein Complexes

• Mitochondrial translocons are complex, multiprotein structures, with each subunit having a specific job.

• Mutations or deficiencies in specific translocon components can lead to various mitochondrial-related syndromes.

• For instance, issues with complexes like Tom or Tim subunits can disrupt protein import and compromise mitochondrial function.

Summary of Energy Inputs for Mitochondrial Protein Import

There are three main energy inputs for moving proteins from the cytosol to the mitochondrial matrix:

1. ATP Hydrolysis (Cytosol): For chaperone unloading and maintaining the protein in an unfolded state before it enters Tom.

2. Membrane Potential (Inner Mitochondrial Membrane): The electrochemical gradient across the IMM drives the positively charged signal sequence through Tim.

3. ATP Hydrolysis (Matrix): For chaperone loading (molecular ratchet mechanism) in the matrix, which helps pull the protein into the matrix and prevents backsliding.