Molecular Genetics

Newly synthesized polypeptides contain amino acid sequences that act as baggage tags directing the polypeptide to its proper destination.

Facts

• The identification of the tags has relied heavily on analysis of translational fusions. Characterized tags include:

• Enzymes (and genes encoding them) have been identified that remove the peptide tags after the polypeptides reach their destination:

• The sequence recognized by the signal peptidase is complex, consisting of three regions:
  • n-region: 1 to 5 amino acids, positively charged
  • h-region: 7 to 15 amino acids, hydrophobic
  • c-region: 5 to 7 amino acids; small residues at -1 and -3 relative to cleavage site.
• Most mitochondrial intermembrane space proteins are partially imported into the matrix with the IMS sorting signal acting as a stop transfer sequence. Other intermembrane proteins are first imported entirely into the matrix and then exported to the intermembrane space.
• Thylakoid proteins are first imported into the stroma and then transferred across the thylakoid membrane.

Interpretations

• Many peptidases consist of several polypeptides, thus requiring several genes to encode them.
• Each distinguishable cellular destination has its own signal or complex of signals.

Further information

• Other peptide sequences, not removed after the protein reaches its destination, guide proteins to other organelles, such as nuclei. Non-amino acid signals guide proteins to lysosomes.
• Targeting to plastid thylakoid membranes resembles targeting to bacterial inner membranes (rev).
• Both outer and inner mitochondrial membranes have translocation complexes, TOM and TIM, respectively. A peptide to be imported is passed directly from TOM to TIM. Two kinds of TIM complexes exist. One specializes in importing integral inner membrane proteins (rev).

This is page 2543 of Molecular Genetics by Ulrich Melcher, © 1998, 1999