Chapter 9. Cell Signaling

Receptors for polar signaling molecules are on the cell surface.

The location of a particular receptor in a cell depends largely on whether the signaling molecule is polar or nonpolar (Fig. 9.6 ). Many signaling molecules, such as the growth factors we just discussed, are small, polar proteins that cannot pass through the hydrophobic core of the plasma membrane. The receptor proteins for these signals are on the outside surface of the responding cell (Fig. 9.6a).

FIG. 9.6 Cell-surface and intracellular receptors. (a) Cell-surface receptors interact with polar signaling molecules that cannot cross the plasma membrane. (b) Intracellular receptors interact with nonpolar signaling molecules that can cross the plasma membrane.

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Receptor proteins for growth factors and other polar ligands are transmembrane proteins with an extracellular domain, a transmembrane domain, and a cytoplasmic domain. When a signaling molecule binds to the ligand-binding site in the extracellular domain, the entire molecule, including the cytoplasmic domain of the receptor, undergoes a conformational change, and as a result the molecule is activated. In this way, the receptor acts as a bridge between the inside and outside of the responding cell that carries the message of the hydrophilic signal across the hydrophobic core of the plasma membrane.