Wk 5.3 Membrane Proteins

Learning Goals

  • Understand transport of integral membrane proteins into the ER membrane.

  • Understand insertion and classification of integral membrane proteins.

  • Understand insertion of proteins into the plasma membrane.

Integral Membrane Proteins

  • Definition: Permanently attached proteins to a membrane.

    • Some parts translocated across the lipid bilayer, others remain embedded in the membrane.

  • Insertion process involves:

    • Signal Recognition Particle (SRP) / SRP receptor pathway (similar to soluble proteins).

  • Classification based on:

    • Topology: Number of times a polypeptide chain spans the membrane.

    • Orientation: Determined by topogenic sequences that guide correct insertion/orientation » form membrane spanning domain into the membrane.

      • Unique topogenic sequences correspond to transmembrane alpha-helices.

Correct Insertion of Integral Membrane Proteins

  • Insertion determined by charge of amino acids surrounding hydrophobic core:

    • Greater number of positive amino acids at terminal end directs N-terminal into cytoplasm (cytosol).

Classification of ER Integral Membrane Proteins

  • Two main types:

    • Single-pass:

      • Contains cleaved ER signal sequence.

      • Anchored by stop transfer anchor sequence; N-terminus in ER lumen.

    • Integration of single-pass transmembrane protein with internal signal sequence

      • Anchored with signal-anchor sequence

      • Orientation determined by charge around signal sequence

    • Multi-pass:

      • Polypeptide chain spans lipid bilayer multiple times, with segments passing both directions.

Insertion Mechanisms

  • Single-pass insertion with ER signal sequence:

    • N-terminal cleaved and targets ER lumen.

  • Multi-pass insertion:

    • Multiple internal sequences allow the protein to cross the membrane repetitively.

Summary of Membrane Protein Classification

  • Single-pass proteins:

    • N-terminal ER signal sequence is cleaved, internal anchor sequence determines orientation.

    • N-terminus ends in lumen; C-terminus in cytoplasm.

  • Multi-pass proteins:

    • Contain several internal anchor sequences and an internal ER signal sequence driving their topology and orientation.

Translocation of Membrane Proteins

  • Integral membrane proteins translocated across the ER during synthesis and transported to Golgi.

    • Sequential transport through the Golgi apparatus:

    • Cis → Medial → Trans to reach plasma membrane.

Discussion Questions

  1. How are integral membrane proteins classified?

    • A: Topology

    • B: Orientation of membrane spanning segments

    • C: Amino acid sequence

    • D: Both A and B

  2. How is the insertion direction of an integral membrane protein determined?

    • A: More positive amino acids at terminal end inserted into the cytoplasm

    • B: More negative amino acids at terminal end inserted into the cytoplasm

    • C: C-terminus always in cytoplasm

    • D: C-terminus always in lumen

Additional Discussion Questions

  • Predict topology and orientation with given amino acid sequences.

  • Determine if the signal sequence is cleaved based on amino acid configuration.

Summary of Key Concepts

  • Integral membrane proteins are classified as single-pass or multi-pass based on their structure and orientation.

  • Correct localization involves differentiation of their pathway through the secretory system.

Further Reading

  • Alberts et al., Molecular Biology of the Cell, 6th Edition, Chapter 12

  • Alberts et al., Essential Cell Biology, 3rd Edition, Chapters 11 and 15