Download CELB30090 Advanced Cell Biology Prof. Jeremy C

CELB30090
Advanced Cell Biology
Prof. Jeremy C. Simpson
Lecture 2
The endomembrane system
Today’s lecture ...
Overview of internal membranes
Importance of subcellular compartmentalisation
Introduction to the ER
The Golgi complex
The endosomal‐lysosomal system
Peroxisomes
The endomembrane system in cells
The endomembrane system in cells
Key points about subcellular membranes and their organisation
The segregation of distinct biochemical reactions into separate membrane enclosed entities (organelles) results in a dramatic enhancement in efficiency
A typical animal cell contains ca. 10 billion protein molecules that need to be co‐ordinated if the cell is to function correctly
Allows conflicting reactions to be carried out in the same cell (eg: protein synthesis and protein degradation)
Cells have evolved mechanisms that allow compartments to communicate with each other
Proteins requiring transport between compartments must have appropriate signals that can be read by the cell
Division into subcellular compartments allows pathways to be established (eg: secretory pathway)
Internal protein transport pathways in cells
Gated transport
Via protein channels or pores
eg: nuclear import and export
Lecture 12
Transmembrane transport (translocation)
Via protein translocation channels
eg: protein entry into the ER
Lecture 3
Vesicular transport
Via membrane ‐bounded intermediates
eg: transport from ER to Golgi
Lectures 4‐7
The endomembrane system in cells
The endomembrane system occupies a significant cell volume
Relative volumes occupied by the major intracellular compartments in a liver cell (hepatocyte)
The endomembrane system in cells
Cell type and role determines relative abundance of organelles
Relative amounts of membrane types in two kinds of eukaryotic cells
The endomembrane system in cells
Lipid composition is highly variable between organelles
Possible evolution of the endomembrane system in eukaryotic cells
Invagination of the plasma membrane
Pinching off would result
in a double membrane
surrounding the ‘nucleus’
‘ER’ would be continuous
with the ‘nuclear envelope’
Engulfment of
a bacterium
Mitochondria have
separate genome
Mitochondria lumen
remains isolated from rest of endomembrane
system
The endomembrane system in cells
Endoplasmic reticulum (ER) (Lecture 3)
Smooth ER – lipid synthesis, steroid hormone synthesis*, detoxification, storage of calcium ions
Rough ER – protein synthesis, translocation, folding, glycosylation, antigen processing*
Golgi complex / apparatus (Lecture 2)
protein modification by glycosylation, completion of sphingomyelin and glycolipid synthesis
Endosomes (Lecture 7)
protein sorting between endo‐ and exocytic routes, sorting of receptors and ligands, signalling Lysosomes (Lectures 2 and 7)
degradation processes, organelle turnover, antigen processing*, fertilisation* Peroxisomes (Microbodies) (Lecture 2)
synthesis and degradation of hydrogen peroxide, oxidation of fatty acids, synthesis of plasmalogens*, photorespiration in plants * in specific cell types
Co‐ordination of the endomembrane system using vesicular transport
Secretory pathway / Biosynthetic pathway / Anterograde pathway / Exocytic pathway
Endocytic pathway / Retrograde pathway
The Golgi complex
discovered by Camillo Golgi (Italian biologist, b. 1843) ‐ inventor of new types of staining procedures that he hoped might reveal the organization of nerve cells within the central nervous system
applied a silver nitrate‐based stain for several days to cerebellum nerve cells and saw darkly staining reticular network near the cell nucleus (Nobel Prize in 1906)
characteristic morphology ‐ flattened, disk‐like membranous cisternae with dilated rims and associated vesicles & tubules
cisternae, typically 0.5 ‐ 1.0 µm diamater, are arranged in orderly stack like pancakes
usually <8 cisternae are present per stack, but may have a few to several 1000 distinct stacks per cell, depends on cell type
mammalian cell Golgi stacks are interconnected by membranous tubules to form a single, large ribbon‐like complex situated adjacent to the cell's nucleus
10 µm
The Golgi complex
Golgi cisternae polarized ‐ cis face (entry face closest to ER)
‐ trans face (exit face at opposite end of stack; closer to plasma membrane)
new materials enter cis face and pass to trans face where they exit Golgi complex from the trans Golgi network (TGN)
mechanism of transport through the Golgi is still strongly debated (Lectures 4 and 5) intrinsically linked to the cytoskeleton, motor proteins and matrix proteins
The Golgi complex
‐ receives newly synthesised proteins from the ER, and processes them by proteolysis, amino acid modification (eg: hydroxylation), and by modifying their carbohydrate chains
‐ as glycoproteins pass though cis & medial Golgi cisternae, most of the mannose residues are removed from the core oligosaccharides
‐ other sugars are added sequentially by various glycosyltransferases (eg: N‐acetylglucosamine)
‐ sialyltransferase is only found in trans end of Golgi stack and adds sialic acid residues to the terminal of the chains
‐ the Golgi complex is wholly responsible for O‐linked glycosylation events
Endosomal‐lysosomal system
‐ provides a link between the secretory and endocytic pathways
‐ major role in cargo sorting, recycling and degradation (Lecture 7) endosome carrier vesicle /
multivesicular body)
Gruenberg J and Stenmark H (2004) Nat Rev Mol Cell Biol 5:317‐323
Lysosomes
‐ typically contain at least 50 different hydrolytic enzymes made in RER and targeted for lysosomes
‐ can hydrolyze virtually every type of biological macromolecule, resulting in low MW products that can be transported across the lysosomal membrane into cytosol
‐ acidic interior (pH 4.6) maintained by proton ATPase pumps
‐ lysosomal morphology is neither distinctive nor uniform, they are dynamic organelles capable of rapid fusion and
fission, often clustered in the peri‐nuclear area of the cell
‐ highly variable size range (25 nm to > 1 µm diameter)
10 µm
Lysosomal functions
‐ general role in degradation of materials brought into cell ‐ specialised role in fertilization ‐ sperm head contains specialized lysosome (acrosome)
‐ organelle turnover (autophagy)
Peroxisomes
‐ surrounded by only a single membrane, and do not contain DNA or ribosomes
‐ typical diameter of 0.1 – 1.0 µm
‐ present in all eukaryotic cells
‐ contain high concentrations of oxidative enzymes, such as catalase and urate oxidase, that are used to detoxify cells
‐ synthesis of plasmalogens – important phospholipid component of myelin sheaths
10µm
Light microscope
Electron microscope
Peroxisomes
‐ contain high concentrations of oxidative enzymes, such as catalase and urate oxidase, that are used to detoxify cells
‐ oxidation reaction is particularly important in liver and kidney cells, where the peroxisomes detoxify various toxic molecules that enter the bloodstream
‐ ca. 25% of the ethanol we drink is oxidised to acetaldehyde in this way
phenols
formic acid
formaldehyde
alcohols
Peroxisomes
peroxisome biogenesis is not yet fully understood – likely a combination of being derived from the ER and self assembly
peroxisome targeting signal: ‐Ser‐Lys‐Leu‐COO‐ (hydroxylated, positive, hydrophobic)
at least 23 ‘peroxins’ so far identified participate in import
defects in peroxin proteins (eg: Pex2) cause severe disease (eg: Zellweger syndrome) Key take home point
Subcellular compartmentalisation allows
eukaryotic cells to carry out more complex
activities with a higher degree of efficiency