Membrane proteins are proteins that interact with, or are part of, biological membranes. They include integral membrane proteins that are permanently anchored to the membrane and peripheral membrane proteins which are only temporarily attached to the lipid bilayer or to integral proteins.

1. MEMBRANE
PROTEINS
MR. RESTY C. SAMOSA
MAEd Biology

2. MEMBRANE PROTEINS
It is any protein that is embedded in the
plasma membrane of the cell. They do
not anchored in one place but tend to
float within the phospholipids bilayer.

3. MEMBRANE PROTEIN

4. PRIMARY FUNCTION OF
MEMBRANE PROTEINS (Campbell,
2004)
1. Attachment to the cytoskeletons and
extracellular matrix
2. Cell signaling
3. Enzymatic activity
4. Transport
5. Intercellular joining
6. Cell- cell recognition

5. Enzymes
Signal
Receptor
ATP
Transport Enzymatic activity Signal transduction

6. Glyco-
protein
Cell-cell recognition Intercellular joining Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)

7. CLASSES OF MEMBRANE
PROTEINS1. Integral Proteins – that penetrate the lipids
bilayer (transmembrane protein).
2. Peripheral Proteins – that are located entirely
outside of the lipid bilayer, on the cytoplasmic or
extracellular side, yet are associated with the
surface of the membrane by noncovalent bonds.
3. Lipids – anchored Proteins – that located
outside the lipid bilayer, on the either the
extracellular or cytoplasmic surface, but are
covalently linked to a lipid molecule that is
situated within the bilayer.

8. MEMBRANE PROTEINS

9. MEMBRANE PROTEINS

10. Various ways in which
proteins associate with the lipid bilayer

11. Membrane protein
attachment by a fatty acid chain or a
prenyl group.

12. INTEGRAL PROTEIN
 Most integral membrane
proteins function in the
following capacities:
 as receptors that bind
specific substances at the
membrane surface,
 as channels or transporters
involved in the movement of
ions and solutes across the
membrane,
 as agents that transfer
electrons during the
processes of
photosynthesis and
respiration.

13. TYPES OF INTEGRAL PROTEINS
Type I: Single transmembrane
span, N-terminus in ectodomain,
C-terminus in cytosol
Type II: Single span, C-terminus in
the ectodomain, N-terminus in
the cytosol
Type III: Multiple spans
Type IV: Several different
polypeptides assembled to form a
channel
Type V: Lipid-linked protein
Type VI: Proteins with both
transdomain component and lipid
anchor

14. DISTRIBUTION OF INTEGRAL PROTEINS:
FREEZE-FRACTURE ANALYSIS
The concept that proteins
penetrate through membranes,
rather than simply remaining
external to the bilayer, was
derived primarily from the results
of a technique called freeze
fracture replication

15. In Most Transmembrane Proteins, the
Polypeptide Chain Crosses the Lipid Bilayer
in an α-Helical Conformation
A segment of a membrane-spanning polypeptide chain
crossing the lipid bilayer as an α helix.
Using hydropathy plots to
localize potential α-helical membranespanning
segments in a polypeptide
chain.

16. Two short α helices in then aquaporin water
channel, each of which spans only halfway
through the lipid bilayer.

17. Converting a single-chain multipassprotein
into a two-chain multipass protein

18. Steps in the folding of a
multipass transmembrane
protein

19. β barrels formed from
different numbers of β strands.

20. A single-pass transmembrane
protein

21. PROPERTIES OF INTEGRAL PROTEINS
Because of their hydrophobic
transmembrane domains,
integral membrane proteins are
difficult to isolate in a soluble
form. Removal of these proteins
from the membrane normally
requires the use of a detergent,
such as the ionic (charged)
detergent SDS (which denatures
proteins) or the nonionic
(uncharged) detergent Triton X-
100 (which generally does not
alter a protein’s tertiary
structure).

22. The use of mild nonionic
detergents for solubilizing, purifying,
and reconstituting functional membrane
protein systems.
functional Na+-K+ pump molecules are
purified and incorporated into
phospholipid
vesicles. This pump is present in the
plasma membrane of most animal cells,
where it uses the energy of ATP
hydrolysis
to pump Na+ out of the cell and K+

23. The carbohydrate layer on
the cell surface.

24. Membrane
protein reconstituted into a
nanodisc.
When detergent is removed from a
solution containing a multipass
membrane protein, lipids, and a
protein subunit of the high-density
lipoprotein (HDL), the membrane
protein becomes embedded in a
small patch of lipid bilayer, which is
surrounded by a belt of the HDL
protein. In such nanodiscs, the
hydrophobic edges of the bilayer
patch are shielded by the protein
belt, which renders the assembly
water-soluble.

25. PATCHES OF PURPLE MEMBRANE, WHICH CONTAIN
BACTERIORHODOPSIN IN THE ARCHAEON
HALOBACTERIUM SALINARUM.

26. AN EXPERIMENT DEMONSTRATING THE DIFFUSION
OF PROTEINS IN THE PLASMA MEMBRANE OF
MOUSE– HUMAN HYBRID CELLS.

27. How membrane molecules can be restricted
to a particular membrane domain.

28. Four ways of restricting the lateral mobility
of specific plasma membrane proteins.

29. LIPIDS – ANCHORED
PROTEINS
Lipid-anchored proteins (also known
as lipid-linked proteins)
are proteins located on the surface
of the cell membrane that
are covalently attached
to lipids embedded within the cell
membrane. These lipids insert and
assume a place in the bilayer
structure of the membrane
alongside the similar fatty acid tails.
The lipid-anchored protein can be
located on either side of the cell
membrane. Thus, the lipid serves to
anchor the protein to the cell
membrane

30. Lipid-linked membrane
proteins.
Some types of protein lipidation:
importance is in localization of
specific proteins to the membrane
(i.e., during signal transduction
pathway activation)

31. GENERAL REFERENCES
Alberts, et al. (2015). Molecular Biology of the Cell (6th ed.). New
York, U.S.A.: Garland Science, Taylor and Francis Group., p. 576 - 594
Becker, et al. (2003). The World of the Cell (5th Ed). New York,
U.S.A.: Pearson Education - Benjamin Cummings., p. 175 – 190.
Campbell, et al. (2004). Essential Biology (2nd Ed). Pearson
Education South Asia Ptd Ltd., p.54.
Lodish, et al. (2013). Molecular Cell Biology (7th ed.) New York,
U.S.A.: W. H. Freeman and Company
Karp, Gerald. (2010). Cell and Molecular Biology: Concepts and
Experiments (6th Ed). New York, U.S.A.: John Wiley & Sons Inc., p. 127
– 143.
Nelson et al. (2008). Lehninger: Principles of Biochemistry (4th
ed.)New York, USA..: Worth Publisher., p. 369 -417.
Voet, et al. (2011) Biochemistry (4th ed.)New York, USA..: John
Wiley & Sons Inc., p. 400 - 461