Membrane proteins serve vital functions and can be classified as integral or peripheral. Integral proteins span the lipid bilayer and include transmembrane and monotopic proteins. Peripheral proteins are temporarily attached to the bilayer or integral proteins. Glycophorin A is an integral protein that spans the erythrocyte membrane once, with different regions exposed inside and outside the cell. Porins are beta membrane proteins that form pores to allow diffusion of molecules across bacterial membranes.

1.  A membrane protein is a protein molecule
that is attached to, or associated with the
membrane of a cell or an organelle. More
than half of all proteins interact with
membranes.

3.  Biological membranes consist of a phospholipid
bilayer and a variety of proteins that accomplish vital
biological functions.
• Structural proteins are attached to microfilaments in
the cytoskeleton which ensures stability of the cell.
• Cell adhesion molecules allow cells to identify each
other and interact.
• Membrane enzymes produce a variety of substances
essential for cell function.
• Membrane receptor proteins serve as connection
between the cell's internal and external
environments.
• Transport proteins play an important role in the
maintenance of concentrations of ions.

4. Integral membrane proteins which are
permanently bound to the lipid bilayer
Peripheral membrane proteins that are
temporarily associated with lipid bilayer or
with integral membrane proteins
Lipid-anchored proteins bound to lipid
bilayer bound through lipidated amino acid
residues

5.  Integral membrane proteins are permanently
attached to the membrane.
 They can be defined as those proteins which
require a detergent (such as SDS or Triton X-100)
or some other a polar solvent to be displaced.
 They can be classified according to their
relationship with the bilayer:
 Integral polytopic proteins, also known as
transmembrane proteins, are protein that are
permanently attached to the lipid membrane
and span across the membrane
 Integral monotopic proteins are proteins that
are permanently attached to the lipid membrane
from only one side and do not span across the
membrane

6.  Bitopic protein are transmembrane proteins
that span across the membrane only once.
Transmembrane helices from these proteins
have significantly different amino acid
distributions to transmembrane helices from
polytopic proteins

7.  Peripheral membrane proteins are temporarily
attached either to the lipid bilayer or to integral
proteins by a combination of hydrophobic,
electrostatic, and other non-covalent
interactions.
 Peripheral proteins dissociate following
treatment with a polar reagent, such as a
solution with an elevated pH or high salt
concentrations.
• Integral and peripheral proteins may be post
translationally modified, with added fatty acid
chains which may be anchored in the lipid
bilayer.

8.  Glycophorin A
 Glycophorin A is a glycoprotein(integral membrane protein
in erythrocytes) by mass ~60% carbohydrate, ~40% protein.
 has a single transmembrane α-helix
 Has only 131 aa residues
 These are rich in sialic acid, which gives the red blood
cells a very hydrophilic-charged coat. This enables them to
circulate without adhering to other cells or vessel walls.
 Most of protein (N-terminal portion) on outside of cell,
exposed to water; mainly hydrophilic residues
 Extracellular part of protein also receptor for influenza
virus binding to cells
 C-terminal portion on cytosolic side of membrane,
interacts with cytoskeletal proteins

10.  Porins are beta proteins that cross a cellular
membrane and act as a pore through which
molecules can diffuse.
 Porins are large enough to allow passive
diffusion, i.e., they act as channels that are
specific to different types of molecules
• They are present in the outer membrane of
Gram-negative bacteria and some Gram-
positive bacteria.
• Porins typically control the diffusion of small
metabolites like sugars, ions, and amino
acids.