Membrane proteins unique to vertebrate olfactory cilia: candidates for sensory receptor molecules

In search for olfactory receptor molecules, we carried out comprehensive electrophoretic mapping of membrane proteins in the cilia of frog olfactory epithelium. Seven polypeptides, extracted from isolated cilia by nonionic detergent, were unique to the sensory organelles, compared to nonsensory (respiratory) counterparts. Olfactory cilia contained 3-10 times more membrane-associated protein as compared to respiratory cilia, in agreement with reported densities of freeze-fracture intramembranous particles. Four of the olfactory polypeptides were major constituents of the ciliary membrane, each amounting to greater than 10% of its total protein. Three major and one minor specific polypeptide were glycosylated, whereas membranes of nonsensory cilia were practically devoid of glycoproteins. A clear difference in surface composition was also shown by microscopic visualization of fluoresceinated lectin bound to intact isolated cilia. Two of the olfactory glycoproteins displayed pronounced heterogeneity of apparent molecular weight, which could partly be due to protein sequence diversity, as expected for odorant receptor molecules. The recently described inhibition of odorant-evoked sensory potentials by the lectin concanavalin A is consistent with the hypothesis that one or more of the specific glycoproteins described here plays a role in olfactory reception.