Cell-surface receptors

Extracellular events trigger intracellular signaling

The surface of the plasma membrane is studded with proteins inserted into the bilayer which connect the extracellular space with the cytoplasm. This network of integral membrane proteins enables cells to sense what happens around them. Transmembrane proteins are stabilized by the insertion of a hydrophobic domain(s) into the amphipathic lipid environment of the membrane bilayer (Figure 8.2): since there are no hydrogen donors or acceptors in the membrane, the hydrogen bonding of transmembrane domains needs to be fully satisfied within the main chain of the peptide itself. Membrane-spanning domains fulfill this requirement for internal hydrogen bonding by adopting repeating main chain secondary structures of bundled α-helices or β-barrels in the presence of apolar side chains. Bacteria have exploited this hydrophobic effect by producing pore-forming toxins such as α-hemolysin, which insert tenaciously into human cell membranes via their nonpolar oligomeric β-barrel structures.