G Protein-Coupled Receptors (GPCRs), a large family of proteins that transmit signals via heterotrimeric G proteins, constitute around 30% of all currents drug targets. They are characterized by seven-transmembrane domains and are found in the plasma membrane of cells. The classification of GPCRs is based on their sequence homology and phylogeny, with classes defined by the presence of shared motifs, structures, and signaling pathways. Let's delve into the fascinating world of GPCRs and explore their major classes.
GPCRs are classified into six major families, each with distinct features and functions. These families are named after their first identified members: rhodopsins, secretins, glutamate receptors, adhesion receptors, fungal pheromone receptors, and hippo-family receptors. Each family exhibits unique architectural characteristics, ligand binding properties, and intracellular signaling pathways.
Rhodopsin-like GPCRs (Class A)
The rhodopsin-like class, or Class A GPCRs, is the largest family, comprising over 80% of all GPCRs. This class includes receptors for light, hormones, growth factors, neurotransmitters, and odorants. They are characterized by a conserved 'DRY' motif in transmembrane helix III and a 'NPxxY' motif near the C-terminus intracellularly. Examples include the beta-2 adrenergic receptor and the delta-opioid receptor.
Rhodopsin-like receptors can be further subclasses based on their ligands. For instance,Rhodopsins form Class A1, while peptide receptors make up Class A2. Furthermore, Class A is divided into subfamilies defined by the type of ligand they bind, such as Class A1/PC, A1/1a, and A1/1b.
Class A1: Rhodopsins (Vision)
The rhodopsins comprise the primary photoreceptors in the eye's retina, responsible for initiating the visual response. They are activated by photons, making them light-sensitive GPCRs. Rhodopsin's unique property is itsopsin, a covalent complex between the protein opsin and the chromophore 11-cis-retinal.
Upon light absorption, 11-cis-retinal undergoes isomerization to form all-trans-retinal, leading to a conformational change in the opsin protein. This change activates the G protein transducin (Gt), which in turn activates a cascade of signaling events culminating in visual perception.
Class A2: Peptide receptors
Class A2 receptors bind peptide ligands ranging from short oligopeptides to larger protein hormones. They play a significant role in various physiological processes, such as growth, analgesia, inflammation, and metabolism. Examples include the glucagon receptor and the chemokine receptors.
The peptide receptors' large extracellular domains allow several interactions with their peptidic ligands, varying from low- to high-affinity binding. This diversity enables a plethora of drug designs and targeted therapies for various diseases.
Secretin-like GPCRs (Class B)
Class B GPCRs, the second-largest family, comprises receptors that bind peptide hormones with long N-terminal extensions. They are characterized by a 'YxxxxP' motif in the third intracellular loop and a 'GWxxA' motif near the C-terminus. Examples include the glucagon receptor and the calcitonin receptor.
Secretin-like receptors require the participation of single-pass membrane proteins called heterodimeric assist proteins (HAPs) for high-affinity ligand binding. This raises intriguing questions about receptor assembly and regulation, opening avenues for novel drug designs.
Class B1: Secretin family receptors
The secretin family consists of receptors that respond to hormones playing roles in digestion, such as secretin and cholecystokinin. These receptors are grouped together due to similar sequences andSimilar signaling mechanisms involving Gs proteins and cAMP production.
One of the evolutionarily conserved features of Class B1 receptors is their N-terminal domain, which forms a large extracellular region crucial for ligand binding. This domain contains secretion family receptor imperiopectrin repeats (SPECRs) that facilitate peptide hormone binding.
Class B2: Metabotropic glutamate receptors (mGluRs)
The metatroptic glutamate receptors (mGluRs) constitute a subclass of ionotropic glutamate receptors. They play a critical role in various neurological processes, such as synaptic transmission and plasticity. There are eight known mGluRs, grouped into three main categories based on their pharmacology and signaling properties.
mGluRs exhibit a unique feature: they possess a large extracellular domain containing a bilobed Venus flytrap (VFT) motif that captures their cognate agonist, L-glutamate. The agonist binding induces a conformational change that activates the G protein coupled intracellular domains, triggering downstream signaling pathways.
Adhesion GPCRs (Class C)
Adhesion GPCRs form a distinct family characterized by an additional extracellular adhesion domain. This family comprises pathways vital for cell-cell communication, adhesion, and signaling. They share a conserved 'GPCR Autodock' (GPRAD) domain within their extracellular adhesion domain.
Adhesion GPCRs are further divided into subfamilies based on the nature of their ligands and additional domains present. For instance, the CD97/GPRP subfamily members contain improtan and EGF-like domains, while the GPR56/124 subfamily members possess a lamina-like extracellular domain.
From vision to pain perception, GPCRs are involved in nearly every physiological process. Their classification based on sequence and structural similarities has elucidated their functions, mechanisms, and drug development. This understanding has paved the way for discovering novel therapeutics, targeting diseases such as diabetes, cancer, and Treasury of horizon brings new opportunities for paradigm-shifting discoveries.