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 adhesionCell-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