The Endomembrane System

Goal of Chapter

• Understanding the Endomembrane System and its components

  • Endoplasmic Reticulum (ER)

  • Rough ER: protein cotranslational translocation

  • Function of Smooth ER

  • The Golgi Complex

  • Structure

  • Transport between ER and Golgi: Anterograde and Retrograde Transport

  • Vesicles: COPII and COPI

  • Exocytosis and Endocytosis (involving clathrin-coated vesicles)

  • Lysosomes

Components of the Endomembrane System

• The Endomembrane System does NOT include chloroplasts or mitochondria.

• Functions of the Endomembrane System:

  • Sites for protein synthesis, processing, and sorting (ER and Golgi complex)

  • Endosomes for carrying and sorting material brought into the cell

  • Lysosomes for digesting ingested material and unneeded cellular components

  • Vesicles for transport

  • Nuclear membrane and cell membrane

• Understanding eukaryotic cells requires knowledge of intercellular membranes and compartmentalization.

• Management of lipid and protein movement between organelles (trafficking) is critical.

The Endoplasmic Reticulum (ER)

• The endoplasmic reticulum consists of continuous networks of flattened sacs, tubules, and vesicles throughout the cytoplasm of eukaryotic cells.

• Structure:

  • Membrane-bound sacs are known as ER cisternae.

  • The intraluminal space is referred to as the ER lumen.

  • Rough Endoplasmic Reticulum (rough ER):

  • Characterized by ribosomes on the cytosolic side of the membrane.

  • Membranes form large, flattened sheets.

  • Smooth Endoplasmic Reticulum (smooth ER):

  • Lacks ribosomes and has various roles in the cell.

  • Membranes form tubular structures.

Protein Synthesis on the ER

• All proteins are initially synthesized on cytosolic ribosomes:

  • Bound ribosomes: Synthesize proteins destined for the ER, Golgi, lysosomes, plasma membrane, or secretion.

  • Cytosolic ribosomes: Synthesize proteins that function in the cytosol, mitochondria, chloroplasts, and peroxisomes.

Overview of Protein Sorting Pathway

• mRNA is translated into polypeptides with distinct signal sequences guiding their destination.

Cotranslational Translocation

• Process of translating proteins targeted for synthesis in the ER lumen:

  • The signal sequence from the polypeptide binds to the Signal Recognition Particle (SRP).

  • This binding pauses translation and escorts the complex to the ER membrane where it binds to the SRP receptor.

  • The ribosome is brought near the translocon, forming an open channel from the cytosol to the ER lumen.

  • SRP is released, using GTP as energy.

  • Signal peptidase cleaves the signal sequence as translation resumes, moving the polypeptide into the ER lumen.

  • Once synthesis completes, the protein remains in the ER lumen, the translocon closes, and the ribosome disassembles.

Single-Pass Proteins

• Involves two sequences for targeting and orienting proteins within the ER membrane:

  • N-terminal signal sequence

  • Stop-transfer anchor sequences (STA, internal sequences).

Topological Classes of Integral Membrane Proteins

• The topology of membrane proteins refers to the number of membrane-spanning segments and their orientation.

• Topogenic sequences (directing membrane insertion and orientation) include:

  • N-terminal signal sequence

  • Stop-transfer anchor sequences (STA)

  • Signal-anchor sequences (SA).

• Internal topogenic sequences remain in mature protein structure as membrane-spanning segments.

• Multipass proteins possess multiple internal topogenic sequences.

Functions of Rough ER

• Involved in biosynthesis and processing of proteins:

  • Ribosomes synthesize both membrane-bound and soluble proteins for organelles of the endomembrane system.

  • Newly synthesized proteins enter the endomembrane system cotranslationally through a pore complex.

  • Functions of Rough ER include:

  • Polypeptide folding

  • Assembly of multimeric proteins

  • Initial glycosylation of glycoproteins

  • Quality control for checking and removing misfolded proteins.

Functions of Smooth ER

• Involved in:

  • Drug detoxification: often through hydroxylation.

  • Carbohydrate metabolism: breakdown of stored glycogen in liver cells.

  • Calcium storage: notably in muscle cells as sarcoplasmic reticulum.

  • Lipid biosynthesis: including phospholipids and cholesterol.

The Golgi Complex

• Structure:

  • Composed of a series of membrane-bounded cisternae.

  • Processes and sorts glycoproteins and membrane lipids from ER for transport.

  • Plays a key role in membrane and protein trafficking in eukaryotic cells.

  • Some cells may contain one large stack, while secretory cells may have many stacks.

Orientation of the Golgi Complex

• The Golgi apparatus has two poles:

  • Cis Golgi: proximal to the ER

  • Trans Golgi: distal and nearer to the plasma membrane.

Overview of Protein Trafficking

• Sorting of proteins initiates in the ER, continuing into early compartments of the Golgi.

• Mechanisms exist to retrieve or retain compartment-specific proteins, with final sorting occurring in the Trans Golgi Network (TGN).

Vesicle Transport Processes

• Coated Vesicles:

  • Most vesicles in protein and lipid transport bear coats of proteins on their cytosolic surfaces.

  • Types of coat proteins include:

  • Clathrin

  • COPI

  • COPII

  • Coat proteins influence vesicle destination and prevent nonspecific membrane fusion.

Types of Coated Vesicles (Table 12-2)

• Clathrin-coated vesicles:

  • Origin: TGN

  • Destination: Endosomes.

• Clathrin-coated vesicles (from plasma membrane):

  • Origin: Plasma membrane.

  • Destination: Endosomes.

• COPI vesicles:

  • Origin: Golgi Apparatus.

  • Destination: ER or Golgi.

• COPII vesicles:

  • Origin: ER.

  • Destination: Golgi.

Vesicle Budding and Membrane Fusion

• Vesicles comprise:

  • GTPase switch proteins

  • Coat proteins

  • Cargo proteins and sorting signals

  • v-SNAREs

• Budding is triggered by small GTP-binding proteins.

  • GTPase activation leads to coat protein recruitment to membrane cargo proteins and their receptors.

  • Targeting sequences on cargo proteins ensure specific interactions with coat proteins during vesicle formation.

• The vesicle is released from the donor organelle and upon hydrolysis of GTP, the protein coat disassembles.

• v-SNAREs in the transport vesicle are critical for identifying the target organelle, engaging with t-SNAREs on target membranes for vesicle fusion.

SNARE Mechanism

• v-SNAREs (vesicle SNAREs) and t-SNAREs (target SNAREs) interact for correct vesicle targeting and fusion, similar to puzzle pieces fitting together.

Anterograde and Retrograde Transport

• Anterograde Transport: Movement toward the plasma membrane; vesicles carry newly synthesized proteins to Golgi or cell surface.

• In the process of exocytosis, vesicles merge with plasma membrane, adding their membrane to it.

• Retrograde Transport: Flow from Golgi back to ER, which allows the retrieval of misplaced proteins and balances lipid flow to the plasma membrane.

• Retrieval involves tags like KDEL (Lys-Asp-Glu-Leu) that mark proteins for retrograde transport.

Exocytosis and Endocytosis

• Exocytosis:

  • Process where vesicles containing secretion products move to the cell surface and fuse with the plasma membrane to release contents outside the cell.

• Endocytosis:

  • Process for internal uptake of extracellular materials by folding inwards from the plasma membrane and pinching off to form an endocytic vesicle.

  • Receptor-Mediated Endocytosis: Special receptors on the cell surface bind specific ligands for internalization (e.g., LDL cholesterol).

Example: LDL Internalization

• Low-density lipoproteins (LDL):

  • Function to transport cholesterol from the liver to cells and can lead to health risks if levels are too high.

• LDL is internalized through receptor-mediated endocytosis where ligands bind to cell-surface receptors, later forming coated vesicles that transport LDL to endosomes.

Lysosomes and Cellular Digestion

• Lysosomes are specialized organelles containing digestive enzymes capable of degrading all major biological macromolecules.

• They maintain an acidic pH (4.0–5.0) inside, aided by ATP-dependent proton pumps.

• Lysosomes have several hydrolase enzymes that help isolate digestive processes from the rest of the cell, ensuring efficient degradation of materials.