Membrane proteins are the binding proteins that mediate conduction of ions or molecules into and out of the cell membrane. It has three major kinds, namely integral, peripheral and lipid-anchored membrane proteins. The membrane protein is the principal constituent of the cell membrane that contributes to the structure of the plasma membrane.
The attachment of membrane protein to the phospholipid bilayer cell membrane can be temporary or permanent. A biological layer has more than hundreds of protein, at defined orientation.
Content: Membrane Proteins
Definition of Membrane Proteins
Membrane protein can define as the constitutive and non-constitutive proteins which attach to the phospholipid bilayer membrane at different locations by either entirely or partially spanning the bilayer. The location of various membrane proteins in the biological cell membrane is responsible for the asymmetry of the membrane and refers as “Membrane sidedness”. Some membrane proteins communicate with the extracellular substances, while some interact with the inner protoplasmic material.
Assembling of Membrane Proteins
As we all know, the proteins form through the translation of mRNA by the association of ribosome and transfer RNA. The ribosome will synthesize either a sequence of true signal or non-cleavable TM segment from its N-terminal. The hydrophobic segment of amino acid residues will travel through the ribosomal tunnel towards the exit site.
Some complex of proteins is present within the cell membrane, which refers as “Translocon” which provides a ribosomal tunnel for the release and insertion of newly synthesized proteins. The ribosome process the release of Tm segments relative to the encoded mRNA sequence. Signal receptor particle (SRP) binds with the incoming polypeptide chain complex, accommodating the signal sequence in the alpha-helical conformation and halts translation.
The SR receptor interacts with the ribosome and SRP and causes the conformational change in SRP, allowing the transfer of the nascent ribosome chain. After the completion of protein synthesis, the ribosome detaches from the translocon, and the released membrane proteins assume their final 3D conformation.
Types of Membrane Protein
These define as the intrinsic proteins that can penetrate all through the phospholipid bilayer membrane, and also refers as “Transmembrane proteins”. Integral proteins have domains within the cytoplasmic and towards the extracellular ends of the lipid bilayer. These can have either single or multiple spanning segments, and contribute about 25-30% of all encoded proteins.
Integral proteins are “Amphipathic” in nature comprising both hydrophilic and hydrophobic regions. The portion residing between the phospholipid bilayer shows hydrophobic character, while the rest of the part protruded outwards shows hydrophilic behaviour. Due to membrane fluidity, these proteins may move laterally within the biological cell membrane. These are firmly attached to the cell membrane.
Thus, the integral proteins are not easily separable, requires the use of ionic and non-ionic detergent like SDS and Triton X-100, respectively. SDS denatures the structure of a protein, while Triton X-100 does not alter the conformation of a protein. Therefore, the solubilisation of proteins by the detergents helps us to study the composition of amino acids, molecular weight etc.
The integral proteins can be Monotopic and Polytopic. Monotopic integral protein consists of single coiled alpha helix, while Polytopic integral protein consists of combinations of coiled alpha-helices. Not all the transmembrane protein contains alpha-helices, few also comprises beta-barrel sheets. Glycophorin-A is the best example of an integral protein found in erythrocytes, comprising 131 amino acid residues and primarily composed of glycoprotein.
The transmembrane protein having alpha helices generally contains 21-26 hydrophobic amino acid residues, which undergoes coiling to form alpha-helix and to facilitate the spanning of the bilayer membrane. Few transmembrane proteins consist of antiparallel arranged beta-strands termed as beta-barrel. Porin beta-barrel contains 16 stranded antiparallel beta-sheet, whose interior is highly hydrophilic with the hydrophobic exterior.
It also refers as “Extrinsic proteins” that associates with the lipid bilayer by weak electrostatic and hydrogen bond interactions. Peripheral proteins are easily separable by the exposure under high pH and treatment with high salt concentration. These are either located directly on the polar heads of the phospholipid bilayer membrane or indirectly on the transmembrane channels.
The peripheral proteins help in anchoring, cell support, and transmission of transmembrane signals. It does not interface with the cell membrane’s hydrophobic core. Peripheral proteins are water-soluble and present higher in number than that of integral proteins. Extrinsic proteins are less mobile.
Lipid Anchored Proteins
It also refers as “Lipid linked proteins” that binds covalently to the lipid membrane either through a fatty acid chain, prenyl group or often via oligosaccharide complex. When the peripheral proteins attach with the lipid membrane through glycosylphosphatidylinositol linkage, then it will refer as “GPI- anchored proteins”.
Lipid linked protein has a characteristic property that it is located on either side of the biological cell membrane. It belongs to the class of “Proteolipids”. Prenylated, fatty acylated and glycosylphosphatidylinositol linked proteins are the three distinct kinds of lipid anchored proteins. These group of proteins may have multiple lipid groups and can function as hydrolytic enzymes, adhesion molecules, receptor proteins etc.
The lipid content of the cell membrane helps in the formation of a semipermeable barrier, but the membrane protein performs rest of other tasks like cell adhesion, signal transduction, ion conduction, cell anchoring and many more tasks.