A polypeptide and a nucleic acid encoding the polypeptide are described. The polypeptide includes a cytotoxic toxin, a targeting domain that selectively binds to a target cell that is a mucus-secreting cell and a translocating domain that translocates the cytotoxic toxin into the target cell. A nucleic acid encoding the polypeptide is also described. Also described is a pharmaceutical composition for topical administration to a patient suffering from mucus hypersecretion which includes the polypeptide and a formulation component selected from the group consisting of an excipient, an adjuvant and a propellant. Methods of treating hypersecretion of mucus, chronic obstructive pulmonary disease (COPD) or asthma are also described. These methods include administering to a patient in need thereof a therapeutically effective amount of the polypeptide.