protein

Integral proteins have domains that extend into the hydrocarbon core of the membrane.
Often they span the bilayer.
Intramembrane domains have largely hydrophobic surfaces, that interact with membrane lipids
Amphipathic detergents are required for solubilization of integral proteins from membranes
? Hydrophobic domains of detergents substitute for lipids, coating hydrophobic surfaces of integral proteins.
? Polar domains of detergents interact with water.
If detergents are removed, purified integral proteins tend to aggregate & come out of solution. Their hydrophobic surfaces associate to minimize contact with water.
? Proteins involved in cell signaling often associate with lipid raft domains.
• Otherwise soluble signal proteins often assemble in complexes at the cytosolic surface of the plasma membrane in part via insertion of attached fatty acyl or isoprenoid lipid anchors into raft domains.
• Integral proteins may concentrate in raft domains via interactions with raft lipids or with other raft proteins.
• Some raft domains contain derivatives of phosphatidylinositol that bind signal proteins with pleckstrin homology domains.
Integral protein structure
Atomic-resolution structures have been determined for a small (but growing) number of integral membrane proteins.
Integral proteins are difficult to crystallize for X-ray analysis.
Because of their hydrophobic
transmembrane domains,
detergents must be present
during crystallization
A membrane-spanning ?-helix is the most common structural motif found in integral proteins.
In an ?-helix, amino acid R-groups protrude out from the helically coiled polypeptide backbone.
The largely hydrophobic R-groups of a membrane-spanning ?-helix contact the hydrophobic membrane core, while the more polar peptide backbone is buried.
Colors: C N O R-group (H atoms not shown).
Cytochrome oxidase is an integral protein whose intra-membrane domains are mainly transmembrane ?-helices.
Explore with Chime the ?-helix colored green at far left.
? If a hydropathy plot indicates one 20-amino acid hydrophobic stretch (1 putative transmembrane ?-helix), topology studies are expected to confirm location of N & C termini on opposite sides of membrane.
If two transmembrane ?-helices are predicted, N & C termini should be on the same side. The segment between the ?-helices should be on the other side
While transmembrane ?-helices are the most common structural motif for integral proteins, a family of bacterial outer envelope channel proteins called porins have instead ? barrel structures.
Porin ?-barrel
A ? barrel is a ? sheet rolled up to form a cylindrical pore.
At right is shown one channel of a trimeric porin complex.