Question

Find information (you may want to search the internet) allowing you to describe a specific example of an extracellular signal that acts through a G protein. Give details about the pathway and make sure to name the original signal molecule and describe the change in cell function. (12 points)

Answer 1

G proteins, also referred to as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a spread of stimuli outside a cell to its interior. Extracellular signalling molecules are cues, like growth factors, hormones, cytokines, extracellular matrix components and neurotransmitters, designed to transmit specific information to focus on cells. G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-protein then interacts with either an ion channel or an enzyme within the membrane. All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site. Rhodopsin may be a seven-pass transmembrane molecule homologous to other members of the G-protein-linked receptor family, and, like its cousins, it acts through a trimeric G protein. The activating extracellular signal, however, isn't a molecule but a photon of light.

In the phosphatidylinositol signal pathway, the extracellular signal molecule binds with the G-protein receptor (Gq) on the cell surface and activates phospholipase C, which is found on the plasma membrane. The lipase hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 binds with the IP3 receptor in the membrane of the sleek endoplasmic reticulum and mitochondria to open Ca2+ channels. DAG helps activate protein kinase C (PKC), which phosphorylates many other proteins, changing their catalytic activities, resulting in cellular responses.

The effects of Ca2+ are also remarkable: it cooperates with DAG in activating PKC and may activate the CaM kinase pathway, within which calcium-modulated protein calmodulin (CaM) binds Ca2+, undergoes a change in conformation, and activates CaM kinase II, which has unique ability to extend its binding affinity to CaM by autophosphorylation, making CaM unavailable for the activation of other enzymes. The kinase then phosphorylates target enzymes, regulating their activities. the 2 signal pathways are connected together by Ca2+-CaM, which is additionally a regulatory subunit of adenylyl cyclase and phosphodiesterase within the cAMP signal pathway.