The human AC133 hematopoietic stem cell antigen is also expressed in epithelial cells and targeted to plasma membrane protrusions

The human AC133 antigen and mouse prominin are structurally related plasma membrane proteins. However, their tissue distribution is distinct, with the AC133 antigen being found on hematopoietic stem and progenitor cells and p rominin on various epithelial cells. To determine whether the human AC133 a ntigen and mouse prominin are orthologues or distinct members of a protein family, we examined the human epithelial cell line Caco-2 for the possible expression of the AC133 antigen. By both immunofluorescence and immunopreci pitation, the AC133 antigen was found to be expressed on the surface of Cac o-2 cells. interestingly, immunore-activity for the AC133 antigen, but not its mRNA level, was down-regulated upon differentiation of Caco-2 cells. Th e AC133 antigen was specifically located at the apical rather than basolate ral plasma membrane. An apical localization of the AC133 antigen was also o bserved in various human embryonic epithelia including the neural tube, gut , and kidney. Electron microscopy revealed that, within the apical plasma m embrane of Caco-2 cells, the AC133 antigen was confined to microvilli and a bsent from the planar, intermicrovillar regions. This specific subcellular localization did not depend on an epithelial phenotype, because the AC133 a ntigen on hematopoietic stem cells, as well as that ectopically expressed i n fibroblasts, was selectively found in plasma membrane protrusions. Hence, the human AC133 antigen shows the features characteristic of mouse promini n in epithelial and transfected non-epithelial cells, ie. a selective assoc iation with apical microvilli and plasma membrane protrusions, respectively , Conversely; flow cytometry of murine CD34(+) bone marrow progenitors reve aled the cell surface expression of prominin, Taken together, the data stro ngly suggest that the AC133 antigen is the human orthologue of prominin.