Membrane Proteins
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Freeze-fracture method
- Freeze cell and fracture it along cell membrane's hydrophobic interior
- Proteins associate with either layer after fracturing
- More proteins associate with cytosolic layer
- Embedded in the bilayer
Transmembrane proteins: amphipathic, pass through both membrane layers
- Single pass or multi-pass
- Alpha helices: hydrophobic side chains
- Beta barrel: multi stranded beta sheet (i.e. porin proteins)
Monolayer associated
- Alpha helix
- Lipid-linked
- Do not extend into the bilayer
- Protein-attached: non-covalently bound to transmembrane protein
- Oligosaccharide-attached: bound to carbohydrate head group of glycolipid
Glycocalyx
- Oligosaccharide side chains and glycolipids form carbohydrate coat on external surface of cell
Membrane protein fluidity
1. Fuse mouse and human cells with surface marker proteins
2. Marker proteins mix on hybrid cell surface
- Conclusion: membrane proteins are fluid
Membrane protein functions
- Transport ions, nutrients and other substances across membrane
- Anchor cells to each other, to extracellular matrix or basement membrane
- Transduce external signals to inside of cell
- Mediate cell-cell recognition of glycoproteins on adjacent cell surfaces
- Enzymatically catalyze metabolic pathways