L21 Membrane Proteins Flashcards
what are the 3 types of membrane proteins?
integral/intrinsic membrane protein which goes all the way through
lipid-linked membrane protein which has a lipid anchor into the membrane
peripheral/extrinsic membrane protein which sits on the surface of the membrane
what are the properties of integral membrane proteins?
Span the membrane with single or multiple transmembrane (TM) segments which are made up predominantly of amino acids with hydrophobic side chains
Interact with fatty acid chains in hydrophobic interior of bilayer
how can integral proteins be extracted from the membrane?
by disrupting the membrane with organic solvents or detergents
describe the structure of Glycophorin A, an integral membrane protein
It consists of a single polypeptide chain (single TM domain) of 131 amino acids in 3 domains:
a hydrophilic domain which is glycosylated and resides on the extracellular side of the erythrocyte plasma membrane;
a short predominantly hydrophobic transmembrane domain which forms an alpha helix;
and a hydrophilic domain that resides in the cytosol of the erythrocyte.
describe the structure of Bacteriorhodopsin, an integral membrane protein
20-30 amino acids in an a-helix required to cross the 45Å thick membrane
Multiple TM domains packed in bundle - 7 transmembrane helices embedded in the hydrophobic interior of the bilayer.
Short loops on either side of membrane
how do lipid-linked membrane proteins stay attached to the membrane?
Several proteins are stably attached to the membrane through direct covalent interaction with lipids, so-called acylated or lipid modified proteins. These include proteins with a glycosyl-phosphatidylinositol (GPI) anchor, such as the prion protein, various viral and cellular proteins that contain myristic acid (myristoylated).
- Proteins are covalently linked to a lipid
- This lipid is inserted in the membrane
- Different proteins use different lipids for attachment
what are the basic structures of lipid-linked prion proteins, viral proteins, signalling proteins and insulin receptors?
prion - on extracellular surface
viral - lipid protein = myristate. in cytosol
signalling - prenylated. farnesyl is lipid protein. in cytosol
insulin receptor - palmitoylated. both extracellular and cytosol. lipids are a helix and palmitate
how are peripheral proteins attached to the membrane, how can they be removed and are they soluble?
Do not interact with hydrophobic core of bilayer
Interact with lipid headgroups or other proteins
Readily removed by high salt solution (NaCl) (ionic strength)
Soluble in aqueous solution
what is the purpose of the cytoskeletal proteins and what are the functions of:
spectrins
ankyrin
actin
band 4.1?
The cytoskeletal proteins are a group of peripheral membrane proteins that form a scaffold on the cytosolic side of the plasma membrane. This extensive network is attached to the cytosolic domains of the integral membrane proteins.
Spectrins - form 200 nm long filaments
Ankyrin - bridges spectrin and band 3 protein
Actin - joins spectrin filaments
Band 4.1 - stabilises spectrin-actin interaction
why is the cytoskeleton important?
important in maintaining shape and rigidity of cell AND in restricting the lateral motion of integral membrane proteins
which hereditary diseases could affect the cytoplasm and what would be the result?
Hereditary spherocytosis and elliptocytosis
- Mutations in genes encoding spectrin or ankyrin
- Result in abnormally shaped erythrocytes
- Degraded more rapidly by spleen
- This results in anaemia
