Why can peripheral membrane proteins be removed from the membrane fraction by tr
ID: 53773 • Letter: W
Question
Why can peripheral membrane proteins be removed from the membrane fraction by treatment with high salt? The integral membrane proteins remain associated with the membrane fractions during the initial steps of the procedure. Even repeated high salt washing will not remove the integral proteins. Why must integral membrane proteins be removed using detergents? Soluble proteins contained with organelles of the endomembrane fraction (ER, Golgi, lysosomes, vacuoles, plasma membrane) are found to be confined to those compartments. Microscopic staining techniques will detect a protein in these organelles, but there will be no evidence that these proteins cross membranes to enter the cytosolic compartment. How can the barrier to trans-membrane diffusion be explained for water-soluble proteins?Explanation / Answer
1. Peripheral proteins or extrinsic proteins have weaker interactions with the membrane.They attach with non covalent interactions such as They are temporarily attached either to lipid bilayer or integral proteins via hydrophobic or electrostatic interactions.The proteins and lipid molecules are held together bynon covalent interactions such as vanderwalls forces ( holds hydrophobic tails) and hydrogen bonding ( binds the hydrophilic heads with water).
They donot interact with the hydrophobic core of the phospholipid bilayer.They are bound to membrane by interactions with integral membrane proteins or directly by interactions with lipid polar head groups.
Therefore these loosely associated proteins on the surface of the membrane can be released by mild treatments that donot involve solubilization of membrane itself.
High salt (1M NaCl)concentration decreases electrostatic interactions between proteins and charged lipids.
Chaotropic ions disrupt hydrophobic bonds present in the membrane surface and promote transfer of hydrophobic groups from non polar environment to aqueous phase.At low concentration ions cause selective solubilization and at high concentration cause protein inactivation.
2.Integral proteins or intrinsic proteins have strong interactions with membrane and are bound permanently with hydrophobic interactions to the phospholipid layer. They contain residues with hydrophobic side chain that interact with fatty acyk group of membrane phospholipid. and also hydrocarbon chain that interact with membrane lipid.
They interact with lipids themselves and in isolation hydrophobic patches will mutually attract resulting in aggragation. Therefore a lipophilic substance usually a detergent is used to prevent aggragation to solubilize protein.
3. Soluble proteins that reside within various compartments are enclosed by various endo membranes, include ER, Golgi complex, endosomes, lysosomes, vescicles, vacuoles. plasma membrane. Endomembrane system contributes to structure and organisation of eukaryotic cell.
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