G Protein-coupled Receptors

The indirect transmission of signals into the cell
is mediated by transmembrane proteins, which
traverse the cell membrane. A first messenger,
e.g., a hormone like epinephrine, triggers an intracellular
reaction by binding to a specific receptor.
This leads to activation of a second messenger,
which in turn initiates a series of reactions
that result in a change of cell function.
Many of the genes for the different proteins involved
in the indirect transmission of signals
are known.