Transmembrane proteins
Transmembrane proteins are proteins which are situated on the lipid membrane of cells. They have transmembrane spanning regions which pass through the lipid bilayer of the cell membranes any number of times depending on the protein in question. There are multiple families of transmembrane proteins and each protein has a specific role. The two main forms of transmembrane proteins are channels and carriers. Channels form a constant pore across the plasma membrane and allow for fast diffusion of molecules. Carriers (or transporters) bind to the solute that is going to cross the membrane and undergo a conformational change so that the binding site closes on the side of the membrane the molecule bound to and opens on the opposite side releasing the substance[1]. Carrier proteins can be divided into three different groups:
Uniporters transport only one molecule across the plasma membrane at a time, unlike symporters and antiporters which transport two different molecules at once (co-transport). However symporters transport two molecules in the same direction across the membrane and antiporters transport the molecules in opposite directions[2].
These proteins are normal highly structured as the amino acid primary protein sequence allows for the formation of alpha-helices or beta-sheets.
For extraction, transmembrane protien require detergent or any non-polar solvent. Some of them (beta-sheets) however can be extracted using denaturing agent.
G Protein Linked Receptors:
This is a form of transmembrane protein consisting of 7 membrane domains and 1 ligand binding domain for example the Glycolipids, the most complex form of these been the gangliosides. Associated to these domains via a binding domain is a trimeric G proteins, consisting of 3 heterologous subunits (α, β, γ). The α-subunit is the catalytically active region, more specifically GTPase[3].
The binding of a signal ligand induces a conformation change within the α-subunit causing GDP to be released, consequently binding GTP. It then dissociates from the connected β & γ units and is now said to be activated.
This goes on to induce many secondary messenger pathways inside cells (cAMP & IP3 cycles)
References:
- ↑ Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Wakter P, 2015, Molecular Biology of the Cell 6th Edition
- ↑ Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell, 6th ed. New York: Garland Science; 2015. Pages 601-602.
- ↑ Alberts, B. Johnson A, Lewis J, Taff M, Roberts K, Walter P, 2007. Molecular Biology of the Cell, 5th edition, Taylor & Francis