Biology Reference
In-Depth Information
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PERIPHERAL
Attached to
Integral protein
PERIPHERAL
Attached to
Membrane lipids
FIGURE 6.4
Peripheral protein attached to an integral protein (left) or to anionic phospholipids (right).
through charges on the surface of integral proteins and those that are attached to the anionic
head groups of phospholipids (
Figure 6.4
).
Peripheral proteins can be removed easily and cleanly from the surface of membranes by
either altering the pH or the media salt concentration. One commonly used method that in
fact often defines peripheral proteins employs release with alkaline carbonate. If the periph-
eral protein is attached via Ca
2
รพ
, a chelating agent will release the protein from the
membrane. By definition, a true peripheral protein exhibits little or no interaction with the
membrane hydrocarbon interior and is removed from the membrane without attached lipids.
There are many well-known examples of peripheral proteins, many of which are equally at
home classified as amphitropic proteins (discussed below). Classic examples of peripheral
proteins include cytochrome c (binds to an integral protein
[7,8]
) and the myelin basic protein
(binds to membrane phospholipids
[9]
).
Cytochrome c is probably the best studied peripheral protein. Its function as an essential
component in the mitochondrial electron transport system where it links Complex III to
Complex IV has been known for many decades. More recently, this very small, primitive
protein has been found to be an example of a 'moonlighting' protein, where it performs an
entirely different function, that of promoting apoptosis
[7]
. Cytochrome c is weakly bound
to cytochrome c oxidase on the outer side (intermembrane space) of the inner mitochondrial
membrane
[8]
. Cytochrome c can be detached from the mitochondrial membrane by simply
washing in 0.15 M KCl. In fact, the protein is often inadvertently lost from the mitochondria
during routine isolation and must be replaced. The facile loss of cytochrome c is what makes
this peripheral protein such a good trigger for apoptosis.
The best example of a peripheral protein that attaches primarily through electrostatic
forces to anionic phospholipids on the surface of a membrane is the myelin basic protein
(MBP)
[9]
. The myelin sheath is considered to be the least dynamic of all membranes and
so has very few proteins. Function of the major myelin proteins, myelin proteolipid protein,
myelin basic proteins, and myelin-associated glycoprotein, is probably structural. MBP is
involved in myelination of nerves in the central nervous system and defects in this protein
are believed to be important in multiple sclerosis.
