By the end of this topic, you should be able to:
• Describe the different functions membrane proteins can perform
• Contrast the different ways in which proteins can be associated with a lipid bilayer
• Describe the diffusion behaviour of proteins in biological membranes
• Explain the practical challenges membrane proteins present for biochemical study
Proteins make up about 50% of the mass of typical biological membranes (Fig 11-4,
p.365). Behaviour of proteins within the lipid bilayer is described by the fluid mosaic
model, which states that proteins are free to diffuse laterally within the bilayer, unless
their movement is restricted by cellular components (Fig. 11-33, p.380).
Functions of membrane proteins (Fig. 11-20, p.372):
1. Transporters: Ions and most polar molecules do not readily cross the
hydrophobic membrane (Fig 12-2, p.389). Transport proteins are required to move
ions and polar molecules between compartments of the cell, or between the inside
and outside. Most transporter proteins are specific for a particular substrate or a
small set of substrates. For example, most cells have potassium channels that
allow K and Rb to d+ffuse ac+oss the membrane, but that are virtually
impermeable to Na and Li .
2. Anchors:Anchor proteins are structural proteins that provide stability to the
membrane and help control the shape of the cell and its position relative to other
cells (Fig 11-31, p.379).Anchor proteins bind to other macromolecules on one or
both sides of the membrane. For example, integrins are anchor proteins that bind
to the extracellular matrix as well as to actin-binding proteins inside the cell.
3. Receptors: These proteins sense chemical signals on the outside of the cell and
carry the message to the inside of the cell, so that the cell can react to its
environment. Like transport proteins, most receptors recognize a very specific
molecular signal. For example, the insulin receptor binds and is activated by
insulin but not other molecules.
4. Enzymes: Proteins that catalyze chemical reactions can be associated with
membranes. Many receptors also have enzymatic activity; for example, after
binding insulin, the insulin receptor relays this signal by adding phosphate groups
to certain proteins inside the cell.
Other examples are given in Table 11-1 on page 373.
Proteins associate with the membrane in four different ways (Fig 11-21, p.373). The
peptide chain of transmembrane proteins completely crosses the membrane at least
once. Some transmembrane proteins have many membrane-spanning segments. The
protein is exposed to both sides of the membrane, and to the hydrophobic interior. The
orientation of the protein with respect to the different sides of the membrane is fixed; that
is, the same side always faces the cytosol. Usually, transmembrane segments of proteins have α-helical secondary structure, and if a
protein has a single transmembrane segment, it is essentially always an α-helix. The
major reason for this is that the α-helix maximizes hydrogen bonding between polar
backbone groups, minimizing interactions between these groups and the hydrophobic
interior of the membrane. Most membrane-spanning α-helices are composed of at least 20
consecutive hydrophobic amino acid residues. The hydrophobic side chains make
favourable interactions with the interior of the lipid bilayer (Fig. 11-23, p.374). The α-
helix does not readily leave the bilayer, because to do so would expose its