Recording-2025-03-04T16:01:23.523Z

Protein Synthesis in Mitochondria

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

• Proteins cannot fully fold into their final conformation during synthesis due to:

  • Requirement to remain linearized for translocation through the translocon.

  • Presence of a signal sequence that must ultimately be cleaved following import.

• Initial form of the protein is termed a pre-protein before it reaches its final structure.

Role of Chaperones

• As proteins are synthesized, they associate with HSP 70 chaperones in the cytosol.

• Chaperones help maintain the linear state of the protein and assist with the proper folding once inside the mitochondrial matrix.

• The signal sequence is essential for recognition by the TOM complex, which has multi-subunit components including receptors.

Signal Sequence Characteristics

• The signal sequence is typically:

  • Helical in shape, located at the N-terminus of the protein.

  • Amphipathic, containing hydrophobic residues and positively charged amino acids.

• Upon recognition by TOM, the protein is translocated into the outer membrane of the mitochondria.

Translocon Complexes: TOM and TIM

• TOM: Translocon of the Outer Membrane. Transports proteins from the cytosol into the mitochondria.

• TIM: Translocon of the Inner Membrane. Further translocates proteins into the mitochondrial matrix or integrates them into the inner membrane.

• Membrane potential plays a significant role in driving signal sequence translocation across TIM.

Protein Destinations within Mitochondria

• Matrix Targeting: If the protein lacks a long hydrophobic sequence, it will enter the matrix, where molecular chaperones assist in proper folding and signal sequence cleavage occurs.

• Membrane Integration: If the protein contains a stop transfer sequence of roughly 20-30 hydrophobic amino acids, it will stop at the TIM lateral gate, embedding into the inner mitochondrial membrane.

• This distinction is critical for the localization of mitochondrial proteins: either in the matrix or as membrane proteins.

Energy Requirements for Translocation

• Energy for the transport mechanism comes from:

  • ATP hydrolysis occurring in the cytosol and matrix during chaperone interactions.

  • The positive charge of the signal sequence creating an electrochemical gradient at the inner membrane.

• The overall directionality of transport is from outside the mitochondria to inside.

Comparison with Bacterial Mechanisms

• Mitochondrial and bacterial protein translocation share similarities, particularly concerning the insertion of beta-barrel proteins into membranes.

• The outer mitochondrial membrane is more permeable due to porin complexes, analogous to porins in gram-negative bacterial outer membranes.

Interaction between Organelles

• Organelles such as the endoplasmic reticulum (ER) can directly connect with mitochondria for lipid transfer, challenging the traditional view of discrete cellular compartments.

• Biomolecular condensates in the cytosol can influence cellular mechanisms and organization.

Chloroplast Import Mechanism

• Chloroplast import operates similarly to mitochondrial protein import, involving a translocon mechanism at both outer and inner membranes.

• Energy inputs are also required during translocation, with potential additional signal sequences directing proteins to specific chloroplast compartments (e.g., stroma vs. thylakoids).

• Proteins for chloroplasts also possess dedicated signal sequences for proper localization.