Is the biophysical tendency of lipids to partition into separate phases within a lipid bilayer functionally utilized in cell membranes? If so, how is it regulated and what membrane functions does it control?
how commonly do specific lipid molecules associate with membrane proteins to regulate their function?
given that the structure of only a tiny fraction of all membrane proteins has been determined, what new principles of membrane protein structure remain to be discovered?
Which statements are true? explain why or why not.
10–1 Although lipid molecules are free to diffuse in the plane of the bilayer, they cannot flip-flop across the bilayer unless enzyme catalysts called phospholipid translocators are present in the membrane.
10–2 Whereas all the carbohydrate in the plasma membrane faces outward on the external surface of the cell, all the carbohydrate on internal membranes faces toward the cytosol.
10–3 Although membrane domains with different protein compositions are well known, there are at present no examples of membrane domains that differ in lipid composition.
Discuss the following problems.
10–4 When a lipid bilayer is torn, why does it not seal itself by forming a “hemi-micelle” cap at the edges, as shown in Figure Q10–1?
seal with hemi-micelle cap
Figure Q10–1 A torn lipid bilayer sealed with a hypothetical “hemimicelle” cap (problem 10–4).
10–5 Margarine is made from vegetable oil by a chemical process. Do you suppose this process converts saturated fatty acids to unsaturated ones, or vice versa? Explain your answer.
10–7 Monomeric single-pass transmembrane proteins span a membrane with a single α helix that has characteristic chemical properties in the region of the bilayer. Which of the three 20-amino-acid sequences listed below is the most likely candidate for such a transmembrane segment? Explain the reasons for your choice. (See back of book for one-letter amino acid code; FAMILY VW is a convenient mnemonic for hydrophobic amino acids.) 10–6 If a lipid raft is typically 70 nm in diameter and each lipid molecule has a diameter of 0.5 nm, about how many lipid molecules would there be in a lipid raft composed entirely of lipid? At a ratio of 50 lipid molecules per protein molecule (50% protein by mass), how many proteins would be in a typical raft? (Neglect the loss of lipid from the raft that would be required to accommodate the protein.) 10–8 You are studying the binding of proteins to the cytoplasmic face of cultured neuroblastoma cells and have found a method that gives a good yield of inside-out vesicles from the plasma membrane. Unfortunately, your preparations are contaminated with variable amounts of right-side-out vesicles. Nothing you have tried avoids this problem. A friend suggests that you pass your vesicles over an affinity column made of lectin coupled to solid beads. What is the point of your friend’s suggestion?
10–9 Glycophorin, a protein in the plasma membrane of the red blood cell, normally exists as a homodimer that is held together entirely by interactions between its trans-membrane domains. Since transmembrane domains are hydrophobic, how is it that they can associate with one another so specifically?
10–10 Three mechanisms by which membrane-binding proteins bend a membrane are illustrated in Figure Q10–2A, B, and C. As shown, each of these cytosolic membrane-bending proteins would induce an invagination of the plasma membrane. Could similar kinds of cytosolic proteins induce a protrusion of the plasma membrane (Figure Q10–2D)? Which ones? Explain how they might work.