G Protein Coupled Receptor
Phagocytosis and pinocytosis are the two basic kinds of endocytosis used by even the most primitive of organisms, but just as exocytosis can be triggered by a signal, there is another category of endocytosis that works a similar way: receptor-mediated. One type of the receptor-based category uses the G protein coupled receptor (GPCR).
Found only in eukaryotes, these receptors are a family of integral membrane proteins – meaning that they are permanently integrated with the plasma membrane – which couple with a special family of proteins known as guanine nucleotide-binding proteins (G proteins) to trigger endocytosis. The G proteins bind with the receptors, and they are activated by a special substance called a ligand. After activation, a signal pathway is formed leading into the cell to induce activity in the cell, like endocytosis. (However, it should be noted that this receptor is not limited to this activity, as its scope goes far beyond it.) Ligands can be a variety of substances, including light-sensitive compounds, odors, hormones (a molecule that triggers activity in a cell), pheromones (a special chemical that induces a social/sexual result in a member of the same species),and neurotransmitters (chemicals that carry information through the neural network to enable center-controlled action, like a brain causing movement in a human). GPCRs are involved in just about every basic and essential organism, body system, activity and faculty, including sight, smell, taste, immune system regulation and the body’s water balance; they can also be involved in negative effects, like the growth of some types of tumors.
GPCRs are essentially a grouping of seven connected transmembrane protein helices, which principally means that they appear as if they are coils (or helices) that are connected to each other by loops or strings of the same protein and their length spans across the thickness of the cell membrane. They could also be seen as passing through the cell membrane seven times, since the seven coils are connected and they are all one protein. GPCRs typically “stick out” of the cell because their helices extend beyond the thickness of the plasma membrane, and this extracellular part can be attached with carbohydrates. Ligands typically bind at the extracellular side of the GPCR while G proteins typically bind at the intracellular side, but this is not always the case. Further, specific types of G proteins, if they have been bound and activated with a GPCR, change the receptor’s affinity for specific types of ligands because G proteins seem to be pre-coupled with their GPCRs—hence the name “G protein coupled receptor.”
Just as exocytosis can be signal-induced, endocytosis can be triggered by a signal pathway created by the binding of certain extracellular and intracellular substances with transmembrane proteins called receptors. One type of receptor, found only in eukaryotes, is called the G protein coupled receptor. This is a transmembrane protein of seven connected helices permanently integrated with the cell membrane. With a specific type of ligand bound to its extracellular side and a typically pre-coupled G protein bound at its intracellular side, the GPCR creates a signal pathway into the cell that triggers activity, such as endocytosis. The GPCR, however, is by no means limited to this activity; in fact, it has a role in just about every basic body function and faculty in the advanced lifeform cells it is present in.