Membrane Proteins

Definition : Proteins that are firmly anchored in the plasma membrane via interactions between its hydrophobic domains and the membrane phospholipids.Also known as intrinsic protein .They include most membrane associated enzymes, antigenic proteins, transport proteins and drug hormone and lectin receptor.

Some membrane proteins are bound only to the membrane surface, whereas others have one region buried within the membrane and domains on one or both sides of it. Protein domains on the extracellular membrane surface are generally involved in cell-cell signaling or interactions. Domains within the membrane, particularly those that form channels and pores, move molecules across the membrane. Domains lying along the cytosolic face of the membrane have a wide range of functions, from anchoring cytoskeletal proteins to the membrane to triggering intracellular signaling pathways.  

Figure :  Transmembrane proteins within lipid bilayer membrane

There are about a thousand receptors in the human genome.These are membrane proteins that bind to chemicals (e.g., drugs) outside of the cell, and this binding process causes a chemical response on the inside of cells.  For example, morphine binds to the mu-opioid receptor in the membranes of brain cells, and the interiors of the cells respond by reducing nerve transmission.

Ion-channels are membrane proteins that allow transport of chemical species into and out of cells.  Nerve transmission, for example, is caused by a difference in ionic strengths across the membrane and this is mediated by ion channels.

Classification of membrane proteins :

Membrane proteins are classified based on the basis of removal of the protein from the membrane .

       

                               1. Integral (intrinsic) Proteins .

                               2. Peripheral (extrinsic) protens.


1. Integral poteins : Integral proteins are very firmly associated with the membrane removal only by agents that interfere with the hydrophobic interactions, such as detergents, organic solvents.

Integral proteins contain sequences rich in hydrophobic hydrocarbons of the lipids , thereby stabilizing the protein-lipid complex.Many integral membrane proteins are glycoproteins.

They act as  transport channels, linkers, receptors, enzymes, structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy, and proteins responsible for cell adhesion. 

2. Peripheral proteins : Peripheral proteins are located on the membrane surface . They associate with the membrane through electrostatic interactions and hydrogen bonding with the hydrophilic domains of integral proteins and with the polar head groups of membrane lipids . Peripheral proteins localized to the cytosolic face of the plasma membrane include the cytoskeletal proteins spectrin and actin in erythrocytes.

They are released by treatment with salt solutions of different ionic strength , extremes of PH; a commonly used  agent is carbonate at high PH. Many peripheral proteins are enzymes and are typical water soluble proteins.

They serve regulators of membrane-bound  enzymes or may limit the mobility of integral proteins by tethering them to intracellular structures.

The peripheral proteins are subclassified on the basis of their mode of attachment to a membrane .

a. Proteolipids : Proteolipids are hydrophobic lipoproteins soluble in chloform and methanol but insoluble in water .They are presents in many membrane but particularly in myelin.

b. Lipophilin : Lipophilin a major lipoprotein of  brain myelin, contains over 65% hydrophobic amino acids and covalently bound fatty acids.

Functions of membrane proteins : Membrane proteins have a variety of functions including ;

1. Membrane proteins act as mediators of transmembrane movement of charged and uncharged molecules.

2. As receptors for the binding hormones and growth factors .

3. Serve as connection between the cells internal and external environments .

4. Act as enzymes involved in transduction of signals.

 5. Some integral membrane proteins have a structural role to maintain the shape of the cell.

6. Transport proteins play an important role in the maintenence of concentrations of ions of cells .These transport proteins come in two forms : Carrier proteins and Channel proteins.

7. They control cell adhesion to form tissues, they dictate development of plants and animals, and they control important metabolic processes, including salt balance, energy production and transmission, and photosynthesis.  In short, they are important in across medicine and agriculture.