Final answer:
A water-soluble hormone binds to a G protein-coupled receptor, as these hormones cannot pass through the cell membrane. The receptor activates a G-protein and triggers an intracellular signaling cascade.
Step-by-step explanation:
A water-soluble hormone would most likely bind to a G protein-coupled receptor (GPCR). Water-soluble or hydrophilic hormones are unable to pass through the lipid bilayer of a cell membrane due to their polarity and therefore require a transmembrane receptor to initiate an intracellular signal. The binding of a water-soluble hormone to the extracellular domain of a GPCR activates an associated G-protein, which in turn may activate other proteins including enzymes or ion channels. This leads to a cascade of intracellular events that carries out the hormone's message inside the cell. GPCR is the interface where extracellular hormones signal to intracellular second messengers such as cyclic AMP (cAMP).