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