In-depth Notes on the Biosynthetic Secretory Pathway

Learning Goals

  • Understand the transport of secretory and organelle proteins across the ER membrane.

  • Learn about the steps in the biosynthetic secretory pathway including:

    • Protein sorting.

    • Compartments proteins pass through.

  • Grasp post-Golgi sorting, maturation, and release of secretory proteins in eukaryotes.

  • Explore the 2 proposed models for protein movement through the Golgi.

ER Membrane Binding via SRP Cycle

  1. Binding of Signal Recognition Particle (SRP):

    • SRP binds to the signal peptide of the growing polypeptide chain, pausing translation.

  2. Recognition of SRP by ER Membrane:

    • SRP binds to its receptor in the rough ER membrane.

  3. Translation Resumes:

    • Protein translocator facilitates the entry of the chain into the ER lumen.

Protein Sorting Signals

  • Key Characteristics of Sorting Signals:

    • Comprised of 15-60 amino acids, often located at the N-terminus.

    • Can be cleaved by signal peptidase post delivery.

    • Include internal signals such as nuclear localization signals.

  • Importance of Hydrophobicity and Charge:

    • These are more critical than the precise amino acid sequence for proper targeting.

    • Signals can include positive, negative, or hydrophobic characteristics.

Protein Entry into ER Lumen

  • Proteins destined for secretion or organelles enter the ER lumen and undergo:

    • Correct folding and multimeric protein assembly.

    • Formation and rearrangement of disulfide bonds, facilitated by protein disulfide isomerase (PDI).

Glycosylation in the ER

  • Most synthesized proteins in the rough ER undergo glycosylation:

    • Oligosaccharide chains are added in the ER lumen and modified in the Golgi apparatus.

    • Special glycosylation via GPI anchor attaches to the C-terminus, enabling membrane insertion and extracellular release.

Biosynthetic Secretory Pathway Overview

  • Pathway Flow:

    • ER → Golgi apparatus → final destinations (e.g., lysosomes, cell surface).

    • Only correctly folded and assembled proteins can exit the ER.

Protein Transportation Mechanisms

  1. Gated Transport:

    • Utilized between nucleus and cytoplasm through nuclear pore complexes.

  2. Transmembrane Transport:

    • Involves translocators moving proteins into distinct areas like mitochondria or the ER.

  3. Vesicular Transport:

    • Membrane-bound vesicles ferry proteins between compartments.

Vesicular Movement and Protein Sorting

  • Vesicles bud from the ER and travel to the Golgi:

    • Vesicular Tubular Clusters:

    • After vesicles fuse, they travel along microtubules carrying proteins.

    • Coating Determinants:

    • COPII coats vesicles for ER to Golgi transport; COPI for retrieval.

Golgi Apparatus Functionality

  • The Golgi consists of flattened vesicles (cis, medial, and trans regions) where:

    • Proteins undergo post-translational modifications including oligosaccharide processing.

    • Proteins are sorted based on signaling for final destinations.

Protein Secretion from Golgi

  • Secretory proteins can be continuously secreted or regulated in response to stimuli (e.g., insulin).

  • Proteolytic cleavages convert proproteins into active forms (example: proinsulin to insulin).

Summary of Protein Secretion Steps

  1. Proteins synthesized on ribosomes (both membrane-bound and free).

  2. Translocation during synthesis involving SRP, receptor, translocon.

  3. Modifications in the ER: Disulfide bonds, folding, glycosylation, assembly.

  4. Transport to the Golgi for further processing.

  5. Protein exit at the trans-Golgi for secretion or delivery to organelles.

Additional Reading

  • Alberts et al. "Molecular Biology of the Cell." 6th Edition, Chapters 12, 13.

  • Alberts et al. "Essential Cell Biology." 3rd Edition, Chapter 15.

Understanding of the process of transport of secretory and particular organelle proteins across the ER membrane

Understanding of the steps involved in the biosynthetic secretory pathway, including the sorting of proteins, all of the compartments proteins pass through

Understanding the post-Golgi sorting, maturation and release of secretory proteins in eukaryotes and the 2 proposed models for how proteins move through the Golgi