Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MEMBRANE PROTEINS
583
(A)
carbohydrate layer cytosol nucleus plasma membrane
Figure 10–25 The carbohydrate layer
on the cell surface. (A) This electron
micrograph of the surface of a lymphocyte
stained with ruthenium red emphasizes the
thick carbohydrate-rich layer surrounding
the cell. (B) The carbohydrate layer is made
up of the oligosaccharide side chains of
membrane glycolipids and membrane
glycoproteins and the polysaccharide
chains on membrane proteoglycans.
In addition, adsorbed glycoproteins,
and adsorbed proteoglycans (not
shown), contribute to the carbohydrate
layer in many cells. Note that all of the
carbohydrate is on the extracellular surface
of the membrane. (A, courtesy of Audrey
M. Glauert and G.M.W. Cook.)
200 nm
(B)
= sugar residue
transmembrane
glycoprotein
adsorbed
glycoprotein
transmembrane
proteoglycan
carbohydrate
layer
glycolipid
lipid
bilayer
CYTOSOL
transient cell–cell adhesion processes, including those occurring in lymphocyte
recirculation and inflammatory responses (see Figure 19–28).
Membrane Proteins Can Be Solubilized and Purified in Detergents
In general, only agents that disrupt hydrophobic associations and destroy the
MBoC6 m10.28/10.27
lipid bilayer can solubilize membrane proteins. The most useful of these for the
membrane biochemist are detergents, which are small amphiphilic molecules of
variable structure (Movie 10.4). Detergents are much more soluble in water than
lipids. Their polar (hydrophilic) ends can be either charged (ionic), as in sodium
dodecyl sulfate (SDS), or uncharged (nonionic), as in octylglucoside and Triton
(Figure 10–26A). At low concentration, detergents are monomeric in solution,
but when their concentration is increased above a threshold, called the critical
micelle concentration (CMC), they aggregate to form micelles (Figure 10–26B–D).
Above the CMC, detergent molecules rapidly diffuse in and out of micelles, keeping
the concentration of monomer in the solution constant, no matter how many
micelles are present. Both the CMC and the average number of detergent molecules
in a micelle are characteristic properties of each detergent, but they also
depend on the temperature, pH, and salt concentration. Detergent solutions are
therefore complex systems and are difficult to study.
When mixed with membranes, the hydrophobic ends of detergents bind to the
hydrophobic regions of the membrane proteins, where they displace lipid molecules
with a collar of detergent molecules. Since the other end of the detergent