How do coated vesicles go to the
right place and fuse with the right
membrane?
The “SNARE” hypothesis
SNARE
• Vesicle-SNAP-receptors (v-SNAREs)
• Target-SNAP-receptors (t-SNAREs)
• SNAP=soluble NSF attachment proteins
• NSF=N-ethylmaleimide-sensitive factor
Lysosomes
• Digestive enzymes
• Low pH (4-5)
• Develop from late
endosomes/hydrolases from Golgi
• Activated by lowering the pH
Extracellular components
• Cell walls
• Extracellular matrix
– Bone
– Cartilage
– Connective tissue
Types of molecules in ECM
• Structural proteins—strength/flexibility
– Collagen
– Elastin
• Proteoglycans--matrix
• Adhesive glycoproteins—stick cells to
matrix
– Fibronectins
– laminins
Collagen--strength
Synthesis--fibroblasts
Elastin--flexibility
Glycosaminoglycans
Proteoglycans—hydrated matrix
Adhesive glycoproteins
• Extracellular
– Fibronectins
– Laminins
• Cell surface
– Integrins
Fibronectins
• Group of adhesive glycoproteins
• 2 long linked proteins
• Several binding domains
Fibronectin roles
•
•
•
•
Anchor cells to ECM
Maintain cell shape
Cell movement
Blood clotting
Laminins
• Found in basal laminae
– Special ECM
– Under epithelial cells
– Separates them from connective tissue
• Role of basal laminae
– Support
– Permeability barrier
• Contain
– Type IV collagen
– Proteoglycans
– laminins
Laminins
• Very large proteins
• 3 linked peptides
• Multiple domains
What binds the cells to the ECM?
Integrins
• Groups of transmembrane proteins
• Link cytoskeleton to ECM
• Fibronectin receptor is best known
Cell-cell interactions
Cell-cell adhesion
Cell-cell communication
Cell-cell adhesion
• Cell adhesion molecules (CAMs)
– Lots of them
– Involved in many cellular processes
• Cadherins
– Adhesive glycoproteins
Cell juctions
• Adhesive junctions
– Strong links
• Tight junctions
– Prevent leaks between cells
• Gap junctions
– Forms direct link between cells
Adhesive junctions
•
•
•
•
Desmosomes
Hemidesmosomes
Adherens junctions
Focal adhesions
All contain
- intracellular attachment proteins—link to
cytoskeleton
- transmembrane linker proteins—link the
cells
Desmosomes—rivets between cells
Adherens junctions
• Belt around cell
• Connects to actin, not
tonofilaments
• Look a lot like
desmosomes
• Found in
– Heart
– Epithelial layers
• Oftern form belt
• Called “focal adhesion”
if connects to ECM
Tight junctions
Gap junctions
• Direct electrical connection
• Formed by connexons
– Protein=connexin
• Prominent in muscle and nerve—e.g.
electrical tissues
• Form of cell-cell communication
Cell walls
Plant cell walls
• Cellulose (40%)
• Branched polysaccharides
– Hemicellulose (20%)
– Pectins (30%)
• Extensins--glycoproteins (10%)
• Lignins—woody tissues
– Insoluble aromatic alcohols
– Cross-link to form wood
Plasmodesmata