G protein
Overview
G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from a variety of stimuli outside a cell to its interior. These proteins are involved in various cellular functions, including signal transduction, protein folding, and protein transport.
Structure
G proteins are made up of three subunits: alpha (α), beta (β), and gamma (γ). The α-subunit binds to guanine nucleotides and has a high affinity for guanosine triphosphate (GTP) and guanosine diphosphate (GDP). When a G protein is 'at rest', the α-subunit will bind to GDP. The β and γ subunits are tightly bound to each other and form a functional unit that can move as a single entity.
Function
The primary function of G proteins is to transmit signals from a cell's exterior to its interior. This process begins when a signaling molecule binds to a specific receptor on the cell surface. The receptor then changes shape and activates the G protein, causing it to release GDP and bind GTP. This exchange triggers the separation of the α-subunit from the βγ dimer, allowing both to interact with other proteins within the cell and propagate the signal.
Types of G Proteins
There are four main types of G proteins, classified based on their α-subunit: Gs, Gi/Go, Gq, and G12/13. Each type is associated with a specific set of receptors and has unique effects within the cell.
Gs Proteins
Gs proteins stimulate the production of cyclic adenosine monophosphate (cAMP) by activating adenylyl cyclase. cAMP acts as a secondary messenger and can initiate a series of intracellular events, such as the activation of protein kinase A.
Gi/Go Proteins
Gi/Go proteins inhibit the production of cAMP by inhibiting adenylyl cyclase. They also regulate the activity of ion channels and are involved in the activation of the mitogen-activated protein kinase (MAPK) pathway.
Gq Proteins
Gq proteins activate phospholipase C, which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers the release of calcium ions from the endoplasmic reticulum, while DAG activates protein kinase C.
G12/13 Proteins
G12/13 proteins are involved in the regulation of the cytoskeleton and cell migration. They activate the small GTPase Rho, leading to changes in cell shape and motility.
Clinical Significance
G proteins play a crucial role in numerous physiological processes, and their dysfunction can lead to a variety of diseases. Mutations in G protein-coupled receptors and G proteins have been linked to conditions such as diabetes, cancer, and certain genetic disorders.