Biochemical analysis of an antigen produced by both human sex chromosomes

It has been argued on both evolutionary and functional grounds that genes must be shared by the mammalian sex chromosomes. The only direct evidence for such genes is our previous finding that loci on the human X (MIC2X) and Y (MIC2Y) chromosomes encode a species-specific cell surface antigen recognised by the monoclonal antibody 12E7. These loci map to the regions of the sex chromosomes which pair at meiosis, and MIC2X has been shown to escape X-inactivation. We have used immunoprecipitation and Western blot analysis combined with one- and two-dimensional polyacrylamide gel electrophoresis to compare the products of MIC2X and MIC2Y. The human specific molecule recognised by the 12E7 antibody is a membrane-associated polypeptide of mol. wt. 32.5 kd and pI = 5.0. No difference in size or charge has been detected between X and Y encoded forms of this molecule confirming that MIC2Y is a functional homologue of MIC2X. An intracellular polypeptide of mol. wt. 29 kd and pI = 7.0 present in the cytoplasm of both human and mouse cells is also recognised by the 12E7 antibody.