Practice question

Operational definition of peripheral membrane proteins rests on extraction behavior under conditions preserving bilayer integrity. Integral proteins contain hydrophobic stretches of about twenty residues embedding deeply into acyl core requiring non-ionic detergent for solubilization. Peripheral proteins attach superficially through electrostatic attraction between clusters of lysine and arginine and acidic phospholipids phosphatidylserine phosphatidylinositol phosphate, via calcium bridging mediated by C2 domains, or via specific protein-protein interaction with cytoplasmic tails of integral proteins without penetrating hydrophobic interior. Examples include cytochrome c bound to cardiolipin on mitochondrial intermembrane space surface, spectrin ankyrin complex linked to Band 3 cytoplasmic domain, phospholipase C delta PH domain recruited to PIP2, protein kinase C. Treating sealed right-side-out vesicles with high ionic strength one molar sodium chloride screens electrostatic attraction, one hundred millimolar sodium carbonate pH eleven converts vesicles to open sheets releasing trapped proteins, and EDTA chelates divalent calcium disrupting calcium-dependent binding. After ultracentrifugation, released peripheral pool appears supernatant while integral proteins remain pellet-associated.