The human genome’s largest group of membrane proteins is called G protein-coupled receptors (GPCR), G-protein linked receptors, serpentine receptors, or heptahelical receptors. GPCRs are composed of seven helices and function properly in transmembrane-advanced cell processes that help cells grow and develop.
They are mostly found in eukaryotes, but some prokaryotes have also been seen to have them. GPCRs play a big role in how cells sense and control their environment. They do this by acting as receptors for inner actuators and external ligands.
GPCRs are needed for many physiological functions, such as the development of immune systems and the metabolism of cells. Because of this, they can also be used as drug targets to treat mental and psychological disorders.
Structure
The largest group of protein receptors in the human genome is called GPCRs. They are protein receptors with a seven-helical transmembrane core with a carboxyl-terminus (C-terminus) and an amino-terminus (N-terminus) on the inside and outside of the membrane, respectively. Around 800 different G-protein receptors are written into the human genome. The olfactory receptors comprise a big part of this number (nearly 460 GPCRs).
Classification
Based on how they are ordered, the protein receptors are put into six families:
Class A (the rhodopsin family):
About 80% of all species are in the rhodopsin family, which has 701 members. Most of the structural parts of this family can be found in the alpha group. Only a few proteins in this family have the beta, gamma, and delta forms.
Class B (the family Secretin):
The secretin family has 15 members, each of which has a large N-terminus extracellular domain. Each domain comprises nearly 160 amino acids and is crucial for ligand binding.
Class C (metabotropic glutamate-like receptor family):
Metabotropic glutamate-like receptors, or mGluRs, are a group of 15 receptors attached to the cell membrane by a G protein. These receptors are excitatory neurotransmitters turned on by a metabotropic process that works roundaboutly.
Class D (fungal mating pheromone receptor family):
About 20 protein receptors that only respond to peptide pheromones make up this class. Class D is further divided into two important subfamilies: Ste2 and Ste3. The homology sequence between these two subfamilies differs, so they are put into different groups.
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Class E (cyclic-AMP receptor family):
Class E of the GPCR comprises cyclic adenosine monophosphate (cAMP) receptors from a protozoan amoeba (Dictyostelium discoideum) that are involved in several chemotaxis signaling systems.
Class F (frizzled receptor family):
Class F of the GPCR is made up of the frizzled receptors of Drosophila. They are very important for signaling from hedgehogs and binding of Wnt.
Visualizing and naming
GPCRs are visualized to study the molecular and cellular processes involved in sequestering, regulation of translocation, deterioration of GPCRs, and recycling. GPCRs can be seen using several different methods.
Immune fluorescence
The immune fluorescence (IF) method uses antibodies that target the extracellular receptor domain names or an N-terminal epitope tag to make it possible to see GPCRs in the membranes of living cells. You can see other parts, such as C-terminal epitope tags and intracellular receptor units.
But this needs permeabilization and fixing of the cells. With this method, it is possible to see receptors only in the membranes and subcellular levels. This method can also be used to find both C-terminal as well as N-terminal epitope tags at the same time.
Auto-fluorescent proteins (AFPs)
Fluorescent proteins are a very useful way to study GPCRs. This is a common and noninvasive imaging method often used to monitor trafficking, gene expression, and distribution inside cells.
Improvements to this method, such as photo-activation, photo-switching, fluorescence-detecting sensors, and timers, offer several new ways to study different cellular functions and parameters, down to the level of a single molecule, through fluorescence labeling.
Protein- and peptide-tags
GPCRs can also be seen through the selective addition of fluorophores to genetically encoded protein or peptide tags. This method was made as an alternative to auto-fluorescent proteins (AFPs) and is used in a wide range of in-vivo imaging technology.
Labeling methods can be divided into two main groups
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Enzymatic labeling is a type of covalent labeling that can’t be undone. It has a high level of stability and is better for studying a protein in detail.
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The non-covalent chelation procedure is at the heart of affinity labeling.
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It’s a simple way to label that is very selective and can be found in more than one place in a protein.
Fluorescent ligands
Fluorescent ligands have been used to study how ligands and receptors work together. Usually, antagonists or agonists are used to marking the ligands. Antagonists tend to bind to their correlating GPCR with a lot of strength, offering a better signal-to-noise ratio than agonists.
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