Another major cell-surface signaling route uses a membrane lipid as the source of two second messengers.

Activated receptors can stimulate phospholipase C (PLC). PLC cleaves the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP$_2$) into

• inositol 1,4,5-trisphosphate (IP$_3$), which is soluble in the cytosol;
• diacylglycerol (DAG), which remains in the membrane.

The split creates two spatially different signals from one precursor.

IP3 releases calcium from intracellular stores

IP$_3$ can diffuse through the cytosol and bind IP$_3$-gated Ca$^{2+}$ channels in an intracellular membrane store such as the endoplasmic reticulum.

Channel opening allows Ca$^{2+}$ to move down its electrochemical gradient into the cytosol, producing a transient increase in cytosolic Ca$^{2+}$.

Calcium then binds regulatory proteins and changes their activity. Because resting cytosolic free Ca$^{2+}$ is kept low, a transient increase can carry a strong signal.

DAG remains membrane-localized

DAG stays within the membrane and, together with Ca$^{2+}$ in many contexts, helps activate protein kinase C (PKC). PKC then phosphorylates downstream target proteins.

The pathway therefore branches:

PIP2
 ↓ PLC
IP3          DAG
 ↓            ↓
Ca2+ release  membrane-local signal
      \      /
       downstream protein regulation

Calcium signals must be terminated

Cytosolic Ca$^{2+}$ cannot remain high indefinitely without disrupting many cellular processes. Pumps and exchangers return Ca$^{2+}$ to internal stores or move it out of the cell, while IP$_3$ and DAG are metabolized.

Cells can encode information not only in Ca$^{2+}$ concentration but also in the timing, duration and spatial localization of Ca$^{2+}$ pulses.

This pathway illustrates how cleavage of one membrane lipid can generate a diffusible messenger, a membrane-retained messenger and a transient ion signal that cooperate to regulate downstream proteins.