Notes on Protein Mobilization and Translocation Processes

  • Introduction to Protein Mobilization

    • Discusses the basics of protein mobilization following translation, often coupled to translocation to the endoplasmic reticulum (ER).
    • This process involves ribosome synthesis of proteins that can proceed directly into the ER, which may or may not be universal across eukaryotic cells.

  • Translation and Translocation

    • Proteins are synthesized in the direction from N-terminus to C-terminus as the ribosome reads mRNA from 5' to 3' end.
    • The process can be classified as cotranslation coupled translocation, particularly when targeting soluble proteins to the ER.

  • Signal Recognition Particle (SRP)

    • A signal sequence at the N-terminus is recognized by the SRP, halting translation until the peptide docks with the ER.
    • SRP binds GTP and interacts with an SRP receptor on the ER membrane, leading to GTP hydrolysis and disassociation from the ribosome.

  • Processes of Translocation

    • Cotranslational Insertion: The ribosome synthesizes the polypeptide while translocating it into the ER, which is common in eukaryotes.
    • Post-translational Insertion: Some proteins are translocated into the ER after synthesis, utilizing chaperone proteins like BIP that prevent reversion of the polypeptide during transport.

  • Differences in Energy Use

    • Cotranslational processes rely on GTP through SRP, while post-translational processes utilize ATP hydrolysis managed by chaperones.

  • Integral Membrane Proteins

    • About a third of synthesized proteins are embedded into the membrane, facilitated by the ER where their orientation (topology) is established.
    • The orientation depends on hydrophobic sequences made of nonpolar amino acids.

  • Classes of Membrane Proteins

    • Five classes of membrane proteins include…
    • Type I: Single-pass, N-terminus in the lumen (exoplasmic side) and C-terminus in cytosol.
    • Type II & III: Different orientations regarding the N and C-terminus and sequence codes for insertion.
    • Type IV: Multi-pass proteins looping through the membrane multiple times.
    • GPI anchored proteins: Linked to lipid molecules; functionally distinct from other classes.

  • Signal Anchors and Stop Transfer Sequences

    • Type I proteins contain a signal peptide; Types II and III use signal anchor sequences instead.
    • These sequences ensure selective integration into the membrane while halting translation appropriately.

  • Tail-Anchored Proteins

    • A unique mechanism utilized where the protein is synthesized to completion before insertion.
    • Employs GET proteins instead of SRP and SEC61, enabling recognition of the fully synthesized protein's C-terminus for translocation into the membrane.

  • Conclusion

    • Overall understanding of these processes is crucial as it lays foundational knowledge on how protein orientation and insertion into membranes dictate their future function in cellular systems.
    • The importance of protein structure-function relationships is illustrated, guiding the comprehension of protein interaction with cellular membranes and external environments.