Question

What are G-protein-coupled receptors? Explain their function in regard to how particular regulatory molecules influence different effector proteins in the membrane.

Short answer

G-protein-coupled receptors (GPCRs) are essential transmembrane proteins that transmit extracellular signals, such as hormones and neurotransmitters, into the cell. Upon ligand binding, GPCRs activate intracellular proteins called G-proteins, which consist of three subunits (Gα, Gβ, and Gγ). The activated G-proteins dissociate and interact with various effector proteins in the cell membrane, such as enzymes or ion channels, initiating downstream signaling cascades and leading to diverse cellular responses. The signal is terminated when the Gα subunit hydrolyzes GTP back to GDP and reassociates with the Gβγ complex.

Step-by-step solution

Introduction to G-Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that play a crucial role in the signal transduction in cells. They receive various types of signals, like hormones, neurotransmitters, or sensory stimuli, from outside the cell and initiate an intracellular response.

Role of GPCRs in Cell Signaling

GPCRs are responsible for transmitting the extracellular signals into the cell. Upon binding to their specific ligand (signal molecule), a conformational change occurs in the GPCR, which allows it to activate a type of intracellular proteins called G-proteins.

Activation of G-Proteins

G-proteins are composed of three subunits: alpha (Gα), beta (Gβ), and gamma (Gγ). In their inactive state, a guanosine diphosphate (GDP) molecule is bound to the Gα subunit, while Gβ and Gγ subunits form a complex together. Upon activation by the GPCR, the Gα subunit releases the GDP molecule and replaces it with a guanosine triphosphate (GTP) molecule. This change in the Gα subunit causes it to dissociate from the Gβγ complex.

Influence on Effector Proteins

Once the G-protein subunits are dissociated, they can now interact with various effector proteins in the cell membrane. The Gα subunit, specifically, can modulate the activity of enzymes like adenylyl cyclase or phospholipase C, while the Gβγ complex can also interact with other effectors, like ion channels or certain kinases. By modulating the activity of these effectors, the downstream signaling cascades are initiated, leading to diverse cellular responses.

Termination of the Signal

The GTP molecule bound to the Gα subunit is eventually hydrolyzed back to GDP, which returns the Gα subunit to its inactive state. Consequently, the Gα subunit reassociates with the Gβγ complex, and the G-protein returns to its initial state, ready for another activation by GPCR. In summary, G-protein-coupled receptors are transmembrane proteins that play a crucial role in signal transduction by transmitting extracellular signals into the cell through the activation of G-proteins. These activated G-proteins then modulate the activity of effector proteins in the cell membrane, triggering a wide range of cellular responses.