Post-Golgi/Vacuoles

Key learning •  Nature of lysosomes, lytic and storage vacuoles.

Functions of plant vacuoles. •  Proteins reach lysosomes and vacuoles from the Golgi

through signals on the protein. They differ in plants and animals.

•  Direct pathway from the ER to protein storage vacuoles •  Endocytosis, phagocytosis and autophagy send materials

to the lysosome. •  Receptor endocytosis and transcytosis. •  Establishing cell polarity through endo/exocytosis:

examples epithelial cells and pollen tube growth.

Protein Storage

Lytic

Transport to Lysosomes and Vacuoles

VACUOLES"

Animal lysosomes • Lytic compartment of the cell.

• Have lower pH than cytosol, due to proton pump.

• Contain different lytic enzymes with acidic pH optimum.

• Protects cytoplasm from enzyme activity.

• In plants the equivalent are the lytic vavuoles

Three pathways to the lysosome

Sorting of lytic enzymes into lysosomes- animals

Specificity of M6P marker

• Proteins destined to lysosomes have terminal phosphorylated Mannose residues

• Not every N-linked oligosaccharide receives this modification

Specificity of M6P marker

Specificity lies in amino acid sequence of target protein: “signal patch”.

Plant Vacuoles • Equivalent of animal lysosomes

• Plant and fungal cells (including yeasts) have vacuoles.

• One large central vacuole, additional small vacuoles, especially in developing cells.

• Plants have many types of vacuoles with different functions:

• Lytic

• Storage

• Increasing cell size

• Regulating turgor pressure

• Same cell can have different types of vacuole, such as for storage and digestion.

Plant Vacuoles

The vacuole controls plant cell size • Plant cell can increase volume without changing volume of cytoplasm.

• Weakening of cell wall supports turgor driven cell expansion.

• Cytoplasmic strands connect cortical cytoplasm to perinuclear cytoplasm.

Different pathways to different vacuoles

• Dense vesicles transport storage proteins to protein storage vacuoles (exception: protein bodies). Example: seed-storage globulins

• Clathrin-coated vesicles transport content of lytic vacuoles.

• Protein bodies enter vacuole by autophagy (Example: seed storage prolamin in cereals).

Plant vacuolar sorting signals are amino acid sequences

Often “NPIR” sequence in N-terminal sequences. No specific sequence in C-terminal, hydrophobic, 10-20 aa long. Identified case-by-case by mutagenesis.

Autophagy in animals and plants • Mitochondrion and peroxisome enclosed by membrane.

• Wheat endosperm: protein bodies at different stages of autophagy by pre-vacuolar compartments.

• Immunogold against seed-storage protein

Endocytosis

Receptor-mediated endocytosis Receptors bind to clathrin coated pits at plasma membrane (e.g. LDL Receptor).

Defective LDL receptors: can’t be internalized, increase risk of arteriosclerosis.

In some cases internalization is required for signaling, which occurs in endosomes (e.g. dynamin mutant reduces EGF Receptor signaling).

Cycling of receptors in endosomes Again, pH differences determine binding strength of receptor. In case of LDL, endpoint is lysis in lysosomes. Receptor-containing vesicles cycle back to PM.

Multivesicular Bodies (MVBs) • Generated from early endosomes

• Destined for fusion with late endosomes and/or lysosomes

• No recycling from late endosomes

• Components internalized into MVBs get degraded

Exocytosis - secretion

2 types of secretion in animal cells

Secretion is the “default” pathway. Happens in absence of -KDEL and other trafficking signals.

Some secretion is regulated and fusion of vesicles is triggered by some signal for secretion.

Proteins are aggregated in secretory vesicles

• Protein concentration increases as vesicles form, aggregates form as concentration increases.

• No known “packaging” signal (foreign protein can be packaged)

• Can be detected as electron-dense material in vesicles.

• Examples: insulin in pancreatic cells.

Transcytosis • Transport of receptors from one cell surface to the other.

• Endocytosis into endosomal system, but transported to a different site.

• Contributes to cell polarity

• Example: transport of maternal antibodies from mother’s milk into newborn rat’ bloodstream.

• Abs bind receptor in acidic gut environment, release in more neutral extracellular fluid.

Secretion and Cell Polarity

As discussed: polar exocytosis Transcytosis

Localized secretion at pollen tube tip (similar mechanisms in fungi)

Secretion and Cell Polarity

Polarity of cells Examples from animals:

Epithelial cells, secreting specific compounds only on one surface.

Nerve cells.

Plants: Pollen tubes, root hairs, epidermal cells (waxes), phloem companion cells.

Is polarity established by targeted delivery of membrane components, or by selective retention at specific sites?

Key learning •  Nature of lysosomes, lytic and storage vacuoles.

Functions of plant vacuoles. •  Proteins reach lysosomes and vacuoles from the Golgi

through signals on the protein. They differ in plants and animals.

•  Direct pathway from the ER to protein storage vacuoles •  Endocytosis, phagocytosis and autophagy send materials

to the lysosome. •  Receptor endocytosis and transcytosis. •  Establishing cell polarity through endo/exocytosis:

examples epithelial cells and pollen tube growth.