protein targeting to mitochondrion

Definition

Target type: biologicalprocess

The process of directing proteins towards and into the mitochondrion, usually mediated by mitochondrial proteins that recognize signals contained within the imported protein. [GOC:mcc, ISBN:0716731363]

Mitochondrial protein targeting is a complex process that ensures the delivery of newly synthesized proteins to their correct location within the mitochondrion. This organelle, responsible for cellular respiration, is composed of two membranes: the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM), creating an intermembrane space between them. The mitochondrial matrix, the compartment enclosed by the IMM, houses essential enzymes involved in the Krebs cycle and oxidative phosphorylation.

Proteins destined for the mitochondrion contain specific targeting signals called mitochondrial targeting sequences (MTS), which are typically located at their N-terminus. These MTS are amphipathic, with both hydrophobic and hydrophilic residues, and are recognized by a complex of chaperone proteins in the cytosol.

The journey begins with the import of proteins into the mitochondrial matrix. This process is facilitated by the translocase of the outer membrane (TOM) complex, which acts as a gatekeeper for protein entry into the mitochondrion. The MTS interacts with receptors within the TOM complex, triggering protein translocation across the OMM.

Once inside the intermembrane space, the protein encounters the translocase of the inner membrane (TIM) complex, a multi-protein assembly responsible for transporting proteins across the IMM. The TIM complex is a multifaceted entity, with different sub-complexes facilitating the import of different types of proteins. TIM23, for example, plays a crucial role in transporting proteins to the matrix.

The import process is driven by the electrochemical gradient across the IMM, which is maintained by the proton pump embedded in the membrane. This gradient provides the energy for protein translocation into the matrix. As the protein translocates through the TIM complex, the MTS is cleaved by a matrix-localized peptidase, signaling that the protein has reached its final destination.

While many proteins remain in the matrix, some are further targeted to the IMM, the intermembrane space, or the outer membrane. These proteins contain additional targeting signals that direct them to their specific location. For example, proteins destined for the IMM possess internal targeting sequences recognized by a distinct TIM complex, TIM22, which facilitates their insertion into the membrane.

Importantly, mitochondrial protein targeting is a highly regulated process, ensuring that only the correct proteins are imported. This is achieved through the interplay of chaperones, targeting signals, and translocases. Chaperone proteins maintain proteins in an unfolded state during transport, preventing aggregation and facilitating efficient translocation. The precise targeting signals within each protein dictate their destination within the mitochondrion.

Overall, mitochondrial protein targeting is a complex and essential process for mitochondrial function. It ensures the efficient delivery of proteins to their correct locations, facilitating the intricate biochemical reactions that power cellular life.
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Proteins (2)

Compounds (41)