​Porins, which are normally present on the outer mitochondrial membrane, reach their destination by:
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Correct option is C

Porins are β-barrel proteins located in the outer mitochondrial membrane. They are synthesized in the cytosol and transported to the mitochondria. The TOM (Translocase of the Outer Membrane) complex facilitates their import into the intermembrane space. From there, they are inserted into the outer mitochondrial membrane via the SAM (Sorting and Assembly Machinery) complex. TIM (Translocase of the Inner Membrane) is not involved in their insertion as it primarily transports proteins into or across the inner mitochondrial membrane.

Information Booster:

1. Porins are essential for regulating the exchange of ions and small molecules across the mitochondrial outer membrane.
2. TOM complex mediates protein import into the mitochondria.
3. SAM complex assists in folding and inserting β-barrel proteins into the outer membrane.
4. Proteins synthesized in the cytosol require mitochondrial targeting sequences for proper import.

     5. The inner mitochondrial membrane relies on TIM complexes for protein transport. Porins are synthesized in the cytosol and imported to mitochondria via the TOM and SAM complexes.

     6. Mitochondrial protein import depends on specific translocases like TOM (outer membrane) and TIM (inner membrane).

    7. β-barrel proteins are uniquely handled by the SAM complex for proper folding and membrane integration.

Additional Knowledge:

• Option (a): Direct synthesis of porins on mitochondrial membrane:
  This is incorrect because mitochondria synthesize a small number of their proteins, but porins are not among them. Most mitochondrial proteins are synthesized in the cytosol.
• Option (b): Synthesis on the ER and transport via vesicles:
  Incorrect, as porins are not synthesized in the endoplasmic reticulum (ER). Vesicle-based transport is typical for proteins destined for the secretory pathway, not mitochondrial proteins.
• Option (c): Synthesis in the cytosol, import by TOM complex:
  Correct. Porins are synthesized in the cytosol, imported through the TOM complex, and inserted into the outer membrane via the SAM complex.
• Option (d): Synthesis in the cytosol, import by TIM complex:
  Incorrect, as TIM complexes are specific for the inner membrane and matrix proteins, not β-barrel proteins like porins.