Final answer:
Both odorant receptor and β-adrenergic receptor signaling cascades involve cAMP as a second messenger to propagate intracellular signal transduction.
Step-by-step explanation:
A common step between the signal-transduction cascade mediated by the odorant receptor and the β-adrenergic receptor is the production and action of the second messenger cyclic AMP (cAMP). In both cascades, upon ligand binding, a receptor undergoes a conformational change and activates a G-protein. This leads to the activation of adenylate cyclase, which converts ATP to cAMP. The cAMP then functions as a second messenger to propagate the signal within the cell, leading to a cellular response.