α(2u)-Globulin, the principal urinary protein of the male rat, has extensive sequence homology with many lipid binding proteins. The highest concentration of α(2u)-globulin is found in the preputial gland, a holocrine secretory organ with pheromonal function. Meibomian and perianal glands are two other modified sebaceous glands with holocrine secretory cycles and pleiomorphic peroxisomes capable of synthesizing pheromonal lipids. Immunocytochemical examination shows the presence of α(2u)-globulin in the acinar cells of all three of these modified sebaceous glands. Whereas in the preputial gland all of the acinar cells exhibit immunoreactivity, in the meibomian and perianal glands only selective cells contain α(2u)-globulin. In the case of the preputial gland, in addition to the acinar cells some stratified epithelial cells also were immunoreactive. In the perianal and meibomian glands, keratinocytes lining nearby hair shafts and select cells of accessory oil glands stained for α(2u)-globulin. In situ hybridization with a cloned cRNA probe confirmed the immunocytochemical data. Presence of the α(2u)-globulin mRNA in these glands was also established by Northern blot analysis. Immunoelectron microscopic examination of preputial α(2u)-globulin showed the presence of this protein in secretory granules of various maturational stages. Immunolabeled α(2u) was also found in attached vesicles containing protein and lipid inclusions. The lytic cells were not only loaded with α(2u)-globulin but also contained sharp-edged, irregularly shaped electron-dense granules which stained heavily for this protein. Specific localization of α(2u)-globulin and its mRNA in three pheromone-producing sebaceous glands and its structural homology with known lipid binding proteins indicate a pheromone carrier role of α(2u)-globulin.
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