A novel five-transmembrane hematopoietic stem cell antigen: isolation, characterization, and molecular cloning

Phenotypic analysis of hematopoietic stem and progenitor cells (HSCs) has been an invaluable tool in defining the biology of stem cell populations. We have recently described the production of AC133, a monoclonal antibody (MoAb) that binds to a novel cell surface antigen present on a CD34(bright) subset of human HSCs. This antigen is a glycosylated protein with a molecular weight of 120 kD. Here, we report the molecular cloning of a cDNA encoding this antigen and show that it does not share homology with any previously described hematopoietic or other cell surface antigen(s). The AC133 polypeptide has a predicted size of 97 kD and contains five-transmembrane (5-TM) domains with an extracellular N-terminus and a cytoplasmic C-terminus. Whereas the expression of tetraspan (4-TM) and 7-TM molecules is well documented on mature and immature hematopoietic cells and leukocytes, this 5-TM type of structure containing two large (255-amino acid [aa] and 290-aa) extracellular loops is unique and does not share sequence homology with any known multi-TM family members. Expression of this protein appears limited to bone marrow in normal tissue by immunohistochemical staining; however, Northern analysis suggests that the mRNA transcript is present in a variety of tissues such as the kidney, pancreas, placenta, and fetal liver. The AC133 antigen is also expressed on subsets of CD34+ leukemias, suggesting that it may be an important early marker for HSCs, as well as the first described member of a new class of TM receptors.