MEMBRANE PROTEINS MR. RESTY C. SAMOSA

MAEd Biology

MEMBRANE PROTEINSIt 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.

MEMBRANE PROTEIN

PRIMARY FUNCTION OF MEMBRANE PROTEINS

(Campbell, 2004)1. Attachment to the cytoskeletons and extracellular matrix

2. Cell signaling3. Enzymatic activity4. Transport5. Intercellular joining6. Cell- cell recognition

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

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.

MEMBRANE PROTEINS

MEMBRANE PROTEINS

Various ways in whichproteins associate with the lipid

bilayer

Membrane proteinattachment by a fatty acid chain or aprenyl group.

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.

TYPES OF INTEGRAL PROTEINS

Type I: Single transmembrane span, N-terminus in ectodomain, C-terminus in cytosolType II: Single span, C-terminus in the ectodomain, N-terminus in the cytosolType III: Multiple spansType IV: Several different polypeptides assembled to form a channelType V: Lipid-linked proteinType VI: Proteins with both transdomain component and lipid anchor

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

In Most Transmembrane Proteins, the Polypeptide Chain Crosses the Lipid Bilayer in an α-Helical ConformationA segment of a membrane-spanning polypeptide chaincrossing the lipid bilayer as an α helix.

Using hydropathy plots tolocalize potential α-helical membranespanningsegments in a polypeptidechain.

Two short α helices in then aquaporin water channel, each of which spans only halfway through

the lipid bilayer.

Converting a single-chain multipassprotein into a two-chain

multipass protein

Steps in the folding of a multipass transmembrane protein

β barrels formed fromdifferent numbers of β strands.

A single-pass transmembrane protein

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).

The use of mild nonionicdetergents for solubilizing, purifying,and reconstituting functional membraneprotein systems.

functional Na+-K+ pump molecules arepurified and incorporated into phospholipidvesicles. This pump is present in theplasma membrane of most animal cells,where it uses the energy of ATP hydrolysisto pump Na+ out of the cell and K+

The carbohydrate layer onthe cell surface.

Membraneprotein reconstituted into ananodisc.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.

PATCHES OF PURPLE MEMBRANE, WHICH CONTAIN BACTERIORHODOPSIN IN THE

ARCHAEON HALOBACTERIUM SALINARUM.

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

CELLS.

How membrane molecules can be restricted to a particular membrane

domain.

Four ways of restricting the lateral mobility of specific plasma

membrane proteins.

LIPIDS – ANCHORED PROTEINSLipid-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

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)

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