Resting State Structural Components
Before signal transduction begins, the system is in a resting state consisting of three key elements functionally linked to the plasma membrane:
- Receptor (R). An integral protein that specifically recognizes and binds its primary messenger (e.g., a hormone molecule).
- G protein ($G_{q/PLC}$). A guanine nucleotide-binding anchored protein composed of three subunits: $\alpha$, $\beta$, and $\gamma$. In its inactive state, the $\alpha$-subunit is tightly bound to a guanosine diphosphate (GDP) molecule.
- Phospholipase C (PLC). A membrane-associated enzyme that acts as the primary effector of the entire cascade.
Cascade Initiation: From Receptor to Lipid Hydrolysis
The process is activated when a primary messenger binds to the receptor on the outer surface of the membrane. The receptor conformation changes, significantly increasing its affinity for the G protein. Formation of this complex triggers nucleotide exchange: the $\alpha$-subunit releases GDP and binds GTP.
Binding to GTP alters the structure of the $\alpha$-subunit. It dissociates from the $\beta\gamma$-dimer and travels via lateral diffusion along the membrane to Phospholipase C. Interaction with the active $\alpha$-subunit activates the enzyme.
Activated phospholipase C cleaves its substrate—the membrane phospholipid phosphatidylinositol 4,5-bisphosphate ($PIP_2$). At this point, the single signal splits into two second messengers:
- Inositol 1,4,5-trisphosphate ($IP_3$) — a water-soluble molecule that diffuses into the cytosol.
- Diacylglycerol (DAG) — a hydrophobic molecule that remains in the membrane.
Calcium Mobilization and Intracellular Effects
The generated $IP_3$ diffuses to the endoplasmic reticulum (ER) and binds to specific receptors on its calcium channels. The channels open, and $Ca^{2+}$ ions rush out of the store into the cytosol down their concentration gradient.
Elevating cytosolic calcium levels simultaneously triggers two metabolic pathways:
- Calcium-calmodulin pathway. Calcium ions bind to a special cytosolic protein, calmodulin, which has 4 binding sites. The active complex (calmodulin + 4$Ca^{2+}$) targets inactive $Ca^{2+}$-calmodulin-dependent protein kinases, converting them to an active state. These kinases then use ATP to phosphorylate intracellular enzymes.
- Protein Kinase C (PKC) pathway. Activating this enzyme requires the simultaneous presence of three factors: $Ca^{2+}$ ions, membrane phosphatidylserine (PS) residues, and DAG molecules. Calcium causes cytosolic PKC to migrate to the membrane, where it firmly anchors to PS and DAG, forming an active quaternary complex. PKC phosphorylates intracellular target proteins at serine amino acid residues.
Inactivation of the Inositol Phosphate System
The cellular response must be precise and transient. As soon as the signaling need subsides, inhibitory mechanisms engage:
- G protein inactivation. The $\alpha$-subunit possesses intrinsic GTPase activity. It spontaneously hydrolyzes GTP to GDP, loses its affinity for phospholipase C, and returns to the initial $\alpha\beta\gamma$-GDP complex. Membrane lipid hydrolysis ceases.
- Calcium clearance. Specialized $Ca^{2+}-ATPase$ pumps on the plasma membrane and ER, alongside active antiporter systems ($Na^+/Ca^{2+}$ and $H^+/Ca^{2+}$ exchangers), actively pump calcium out of the cytosol.
- Reversal of phosphorylation. Phosphoprotein phosphatases cleave phosphate groups from previously modified proteins, restoring their original conformation.
- Resynthesis. $IP_3$ and DAG can be recycled to replenish $PIP_2$ stores for future signal transmission.