Peripheral membrane proteins are membrane protein
s that adhere only temporarily to the biological membrane
with which they are associated. These protein
s attach to integral membrane protein
s, or penetrate the peripheral regions of the lipid bilayer
. The regulatory protein subunits of many ion channel
s and transmembrane receptor
s, for example, may be defined as peripheral membrane proteins. In contrast to integral membrane proteins, peripheral membrane proteins tend to collect in the water-soluble component, or fraction, of all the proteins extracted during a protein purification
procedure. Proteins with GPI anchor
s are an exception to this rule and can have purification properties similar to those of integral membrane proteins.
The reversible attachment of proteins to biological membranes has shown to regulate cell signaling
and many other important cellular events, through a variety of mechanisms.
For example, the close association between many enzyme
s and biological membranes may bring them into close proximity with their lipid substrate
Membrane binding may also promote rearrangement, dissociation, or conformational change
s within many protein structural domains, resulting in an activation of their biological activity
Additionally, the positioning of many proteins are localized to either the inner or outer surfaces or leaflets of their resident membrane.
This facilitates the assembly of multi-protein complexes by increasing the probability of any appropriate protein–protein interactions
1. interaction by an amphipathic α-helix
parallel to the membrane plane (in-plane membrane helix)
2. interaction by a hydrophobic loop
3. interaction by a covalently bound membrane lipid (''lipidation'')
4. electrostatic or ionic interactions
with membrane lipids (''e.g.'' through a calcium ion)
Binding to the lipid bilayer
Peripheral membrane proteins may interact with other proteins or directly with the lipid bilayer
. In the latter case, they are then known as ''amphitropic'' proteins.
Some proteins, such as G-protein
s and certain protein kinase
s, interact with transmembrane proteins and the lipid bilayer simultaneously. Some polypeptide hormones
, antimicrobial peptides
, and neurotoxins
accumulate at the membrane surface prior to locating and interacting with their cell surface receptor targets, which may themselves be peripheral membrane proteins.
The phospholipid bilayer
that forms the cell surface membrane consists of a hydrophobic
inner core region sandwiched between two regions of hydrophilic
ity, one at the inner surface and one at the outer surface of the cell membrane (see lipid bilayer
article for a more detailed structural description of the cell membrane). The inner and outer surfaces, or interfacial regions, of model phospholipid
bilayers have been shown to have a thickness of around 8 to 10 Å
, although this may be wider in biological membrane
s that include large amounts of ganglioside
s or lipopolysaccharide
The hydrophobic inner core region of typical biological membranes
may have a thickness of around 27 to 32 Å, as estimated by Small angle X-ray scattering (SAXS)
The boundary region between the hydrophobic inner core and the hydrophilic interfacial regions is very narrow, at around 3Å, (see lipid bilayer
article for a description of its component chemical groups). Moving outwards away from the hydrophobic core region and into the interfacial hydrophilic region, the effective concentration of water rapidly changes across this boundary layer, from nearly zero to a concentration of around 2 M
The phosphate groups within phospholipid bilayers are fully hydrated or saturated with water and are situated around 5 Å outside the boundary of the hydrophobic core region (see Figures ).
Some water-soluble proteins associate with lipid bilayers ''irreversibly'' and can form transmembrane alpha-helical or beta-barrel
channels. Such transformations occur in pore forming toxins
such as colicin
A, alpha-hemolysin, and others. They may also occur in BcL-2 like protein
, in some amphiphilic antimicrobial peptides
, and in certain annexin
s . These proteins are usually described as peripheral as one of their conformational states is water-soluble or only loosely associated with a membrane.
Membrane binding mechanisms
(1poc). Middle plane of the lipid bilayer – black dots. Boundary of the hydrocarbon core region – red dots (extracellular side). Layer of lipid phosphates – yellow dots.
The association of a protein with a lipid bilayer
may involve significant changes within tertiary structure
of a protein. These may include the folding
of regions of protein structure that were previously unfolded or a re-arrangement in the folding or a refolding of the membrane-associated part of the proteins . It also may involve the formation or dissociation of protein quaternary structure
s or oligomeric complexes
, and specific binding of ion
, or regulatory lipids
Typical amphitropic proteins must interact strongly with the lipid bilayer in order to perform their biological functions. These include the enzymatic processing of lipids and other hydrophobic substances, membrane anchoring, and the binding and transfer of small nonpolar compounds between different cellular membranes. These proteins may be anchored to the bilayer as a result of hydrophobic interactions between the bilayer and exposed nonpolar residues at the surface of a protein, by specific non-covalent binding interactions with regulatory lipids , or through their attachment to covalently bound lipid anchors
It has been shown that the membrane binding affinities of many peripheral proteins depend on the specific lipid composition of the membrane with which they are associated.
Non-specific hydrophobic association
Amphitropic proteins associate with lipid bilayers via various hydrophobic
anchor structures. Such as amphiphilic α-helix
es, exposed nonpolar loops, post-translationally acylated or lipidated amino acid residues, or acyl chains of specifically bound regulatory lipids such as phosphatidylinositol phosphate
s. Hydrophobic interactions have been shown to be important even for highly cationic peptides and proteins, such as the polybasic domain of the MARCKS protein
or histactophilin, when their natural hydrophobic anchors are present.
Covalently bound lipid anchors
Lipid anchored protein
s are covalently attached to different fatty acid acyl
chains on the cytoplasm
ic side of the cell membrane
, or prenylation
. On the exoplasmic face of the cell membrane, lipid anchored proteins are covalently attached to the lipids glycosylphosphatidylinositol
(GPI) and cholesterol
Protein association with membranes through the use of acyl
ated residues is a reversible process
, as the acyl chain can be buried in a protein's hydrophobic binding pocket after dissociation from the membrane. This process occurs within the beta-subunits of G-protein
s. Perhaps because of this additional need for structural flexibility, lipid anchors are usually bound to the highly flexible segments of proteins tertiary structure that are not well resolved by protein crystallographic studies
Specific protein–lipid binding
proteins are recruited to different cellular membranes by recognizing certain types of lipid found within a given membrane.
Binding of a protein to a specific lipid occurs via specific membrane-targeting structural domains that occur within the protein and have specific binding pockets for the lipid head groups
of the lipids to which they bind. This is a typical biochemical
interaction, and is stabilized by the formation of intermolecular hydrogen bond
s, van der Waals interactions
, and hydrophobic interactions
between the protein and lipid ligand
. Such complexes are also stabilized by the formation of ionic bridges between the aspartate
residues of the protein and lipid phosphates via intervening calcium
). Such ionic bridges can occur and are stable when ions (such as Ca2+
) are already bound to a protein in solution, prior to lipid binding. The formation of ionic bridges is seen in the protein–lipid interaction between both protein C2 type domains
Protein–lipid electrostatic interactions
Any positively charged protein will be attracted to a negatively charged membrane by nonspecific electrostatic
interactions. However, not all peripheral peptides and proteins are cationic, and only certain sides of membrane
are negatively charged. These include the cytoplasmic side of plasma membrane
s, the outer leaflet of outer bacterial
membranes and mitochondria
l membranes. Therefore, electrostatic interactions
play an important role in membrane targeting
carriers such as cytochrome c
, cationic toxins such as charybdotoxin
, and specific membrane-targeting domains such as some PH domain
s, C1 domain
s, and C2 domain
Electrostatic interactions are strongly dependent on the ionic strength
of the solution. These interactions are relatively weak at the physiological ionic strength (0.14M NaCl
): ~3 to 4 kcal/mol for small cationic proteins, such as cytochrome c
Spatial position in membrane
Orientations and penetration depths of many amphitropic proteins and peptides in membranes are studied using site-directed spin labeling
chemical labeling, measurement of membrane binding affinities of protein mutants
solution or solid-state NMR spectroscopy
ATR FTIR spectroscopy
X-ray or neutron diffraction,
and computational methods.
Two distinct membrane-association modes of proteins have been identified. Typical water-soluble proteins have no exposed nonpolar residues or any other hydrophobic anchors. Therefore, they remain completely in aqueous solution and do not penetrate into the lipid bilayer, which would be energetically costly. Such proteins interact with bilayers only electrostatically, for example, ribonuclease
interact with membranes in this mode. However, typical amphitropic proteins have various hydrophobic anchors that penetrate the interfacial region and reach the hydrocarbon interior of the membrane. Such proteins "deform" the lipid bilayer, decreasing the temperature of lipid fluid-gel transition.
The binding is usually a strongly exothermic reaction.
Association of amphiphilic α-helices with membranes occurs similarly.
peptides with nonpolar residues or lipid anchors can also penetrate the interfacial region of the membrane and reach the hydrocarbon core, especially when such peptides are cationic and interact with negatively charged membranes.
Peripheral enzymes participate in metabolism
of different membrane components, such as lipids (phospholipase
s and cholesterol oxidase
s), cell wall oligosaccharides
), or proteins (signal peptidase
and palmitoyl protein thioesterase
can also digest lipids that form micelle
s or nonpolar droplets in water.
Membrane-targeting domains (“lipid clamps")
170px| [[C1 domain
of PKC-delta (1ptr)
Middle plane of the lipid bilayer – black dots. Boundary of the hydrocarbon core region – blue dots (cytoplasmic side). Layer of lipid phosphates – yellow dots.
Membrane-targeting domains associate specifically with head groups of their lipid ligands embedded into the membrane. These lipid ligands are present in different concentrations in distinct types of biological membranes (for example, PtdIns3P
can be found mostly in membranes of early endosome
in late endosome
s, and PtdIns4P
in the Golgi
Hence, each domain is targeted to a specific membrane.
and phorbol esters
*Pleckstrin homology domain PX domain
and Tubby protein|Tubby domains
bind different Phosphatidylinositol|phosphoinositides
are more specific for PtdIns3P.
bind Phosphatidylinositol (3,4)-bisphosphate|PtdIns(3,4)P2
*Proteins from Merlin (protein)|ERM (ezrin/radixin/moesin) family
-binding proteins include phosphotyrosine
and certain PDZ domain
s. They bind PtdIns(4,5)P2.
*Discoidin domains of blood coagulation
, VHS and ANTH
Structural domains mediate attachment of other proteins to membranes. Their binding to membranes can be mediated by calcium
) that form bridges between the acidic protein residues and phosphate groups of lipids, as in annexins or GLA domains.
Transporters of small hydrophobic molecules
These peripheral proteins function as carriers of non-polar compounds between different types of cell membranes or between membranes and cytosolic protein complexes. The transported substances are phosphatidylinositol, tocopherol, gangliosides, glycolipids, sterol derivatives, retinol, fatty acids, water, macromolecules, red blood cells, phospholipids, and nucleotides.
*Glycolipid transfer protein
s including retinol binding protein
s and fatty acid
*Polyisoprenoid-binding protein, such as YceI protein domain
*Ganglioside GM2 activator protein
sec14p transfer proteins)
*Sterol carrier protein
*Phosphatidylinositol transfer proteins and STAR domains
These proteins are involved in electron transport chain
s. They include cytochrome c
, high potential iron protein
, adrenodoxin reductase, some flavoprotein
s, and others.
Polypeptide hormones, toxins, and antimicrobial peptides
Many hormones, toxins
s, or antimicrobial peptides
interact specifically with transmembrane protein
complexes. They can also accumulate at the lipid bilayer surface, prior to binding their protein targets. Such polypeptide ligands are often positively charged and interact electrostatically
Some water-soluble proteins and peptides can also form transmembrane channel
s. They usually undergo oligomer
ization, significant conformational change
s, and associate with membranes irreversibly. 3D structure of one such transmembrane channel, α-hemolysin
, has been determined. In other cases, the experimental structure represents a water-soluble conformation that interacts with the lipid bilayer peripherally, although some of the channel-forming peptides are rather hydrophobic and therefore were studied by NMR spectroscopy
in organic solvents or in the presence of micelles
* Antimicrobial peptide
* Membrane protein
* Transmembrane proteins
*Seaton B.A. and Roberts M.F. Peripheral membrane proteins. pp. 355–403. In ''Biological Membranes'' (Eds. K. Mertz and B.Roux), Birkhauser Boston, 1996.
*Benga G. Protein-lipid interactions in biological membranes, pp. 159–188. In ''Structure and Properties of Biological Membranes'', vol. 1 (Ed. G. Benga) Boca Raton CRC Press, 1985.
*Kessel A. and Ben-Tal N. 2002. Free energy determinants of peptide association with lipid bilayers. In ''Current Topics in Membranes'' 52: 205–253.
External linksPeripheral membrane proteins
in OPM databaseDOLOP
Genomics-oriented database of bacterial lipoproteinsPeptaibol databaseAntimicrobial Peptide Database
Category:Peripheral membrane proteins