Protein Sorting and Transport: The Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes

Protein Sorting and Transport: The Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes

Introduction

Eukaryotic cells possess a nucleus and membrane-enclosed organelles that facilitate cellular functions despite larger sizes compared to prokaryotes. The efficient organization of cellular functions is achieved through protein sorting and targeting, ensuring proteins reach their designated compartments. Key organelles involved in this process include the endoplasmic reticulum (ER), Golgi apparatus, endosomes, and lysosomes, all interconnected by vesicular transport.

Overview of Organelles

Organelles

  • Cell Membrane Composition:
    • Made up of phospholipids, cholesterol, and proteins.
  • Eukaryotic Organelles:
    • Nucleus
    • Endoplasmic Reticulum (ER)
    • Golgi Apparatus
    • Peroxisomes
    • Mitochondria
    • Chloroplasts
    • Lysosomes (membrane-bound compartments)
  • Membrane Characteristics:
    • Double-membrane organelles: nucleus and mitochondria.
    • Triple-membrane: chloroplast.
    • Remaining organelles: single-membrane structures.

Protein Synthesis and Targeting

  • Types of Proteins:
    • Cytosolic proteins (located in cytoplasm)
    • Organelle-specific proteins
  • Sites of Protein Synthesis:
    • Free ribosomes located in the cytoplasm
    • Rough ER, where ribosomes attach for protein synthesis, targeting proteins for transport to single-membrane organelles.

Chapter 11 Overview

11.1 The Endoplasmic Reticulum

Learning Objectives:

  1. Diagram the secretory pathway aimed at targeting single-membrane organelles or for secretion. (example: exocytosis)
  2. Summarize mechanisms directing proteins to the ER.
  3. Explain insertion of proteins into the ER membrane.
  4. Describe protein folding and quality control within the ER.
  5. Distinguish between the functions of smooth and rough ER.
  6. Explain transport to and retrieval from the Golgi apparatus.

Structure and Function of the Endoplasmic Reticulum

  • Eukaryotic cells contain the endoplasmic reticulum, which is subdivided into two types:
    • Rough ER: Ribosomes on the outer surface facilitate protein synthesis.
    • Smooth ER: Lacks ribosomes, involved primarily in lipid metabolism.

Secretory Pathway Overview

Key Experiments:

  • Palade et al. (1960) developed techniques using radiolabeled proteins to trace the secretory pathway, observing protein movement from the rough ER through Golgi apparatus and eventual secretion from the cell, confirming the fluid mosaic model's principles.

Protein Sorting and Transport

  • Post-Translational Transport: Proteins often undergo export from the ER to their destinations after complete synthesis.

Protein Synthesis Mechanism

  • Isolation of Rough ER:
    • Mechanically disrupt cells, separating smooth and rough microsomes through density-gradient centrifugation.

Insertion and Folding of Proteins in the ER

Signal Sequences and Transition to ER

  1. Signal Sequences: Amino-terminal signal sequences facilitate the recognition and translocation of proteins for insertion into the ER.
  2. Cotranslational Targeting: Proteins with signal sequences engage with Signal Recognition Particles (SRP), which bind to SRP receptors on the ER membrane to promote translocation into the ER lumen.

Protein Folding in the ER

  • Protein folding, assisted by molecular chaperones like BiP, occurs in the ER lumen, where disulfide bonds form in an oxidizing environment, facilitating correct protein conformation.

Glycosylation Processes

  • Glycosylation involves transferring oligosaccharides from dolichol carriers onto proteins during translocation across the ER membrane.
    • Post-glycosylation, some glucose residues are enzymatically removed, resulting in a properly folded and modified protein ready for further trafficking.

Chapter 11 Overview (2 of 4)

11.2 The Golgi Apparatus

Learning Objectives:

  1. Relate structure of the Golgi apparatus to its diverse functions in protein and lipid processing.
  2. Describe glycosylation types occurring in the Golgi.
  3. Enumerate the Golgi's role in lipid synthesis and provide a diagram of protein export routes.

Structure of the Golgi Apparatus

  • The Golgi apparatus is structured with cis, medial, and