Protein Termini and Membrane Insertion😊

Protein Termini

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N-terminus
C-terminus
Amino acid residues at
• Beginning of protein
• End of a protein
Function
• Directs protein to a specific location
• Protein-protein interactions
• Enzymatic activity
Mnemonic
N → begiNNing → sigNal → Navigation
C → End → Enzyme → E (I)nteraction

Protein Insertion into ER Membrane

notion image

Key Proteins that Enter ER

  • Water soluble proteins (enter ER lumen)
  • Transmembrane proteins (insert into ER membrane)

Mechanisms

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Type I
Type II
Type III
In ER lumen
N-terminus
C-terminus
N-terminus
In Cytoplasm
C-terminus
N-terminus
C-terminus
Cleavage of initial signal sequence
By Signal peptidase
No cleavage
No cleavage
Requires
Signal & stop-transfer sequences
Internal signal sequence
Internal signal sequence
  • Type IV (multipass): Requires multiple Internal signal and stop-transfer sequences.

Type I Protein

  • Nascent peptide synthesis begins.
  • A "stop transfer" sequence (1) in the peptide chain reaches the translocon
    • stops peptide transfer into ER lumen.
  • Signal peptidase (2)
    • cleaves the initial signal sequence.
  • The "stop transfer" sequence (3) exits the translocon laterally
    • embeds into the ER membrane.
  • Ribosome continues translating remaining protein into the cytoplasm.
    • Result:
      • Single-pass transmembrane protein.