Investigation of the biochemical relationship between the blood group antigens Xga and CD99 (12E7 antigen) on red cells.

The biochemical relationship between the red cell antigens Xga and the MIC2 gene product, CD99--previously designated the 12E7 antigen--has been examined by immunoblotting and immunoprecipitation analyses of the protein molecules bearing these antigens. Immunoblotting of membrane components and Xga-immunoprecipitates with anti-Xga has shown that Xga antigen is carried on a broad band of apparent molecular weight (Mr)) 24,500-29,500, which consists of a dark stained component at M(r)24,500 and a more diffusely stained component at approximately M(r) 26,500-29,500. Immunoblotting of membrane components and 12E7-immunoprecipitates with 12E7, and RFB-1 and NaM123 which also recognise CD99, distinguished two bands of M(r) 30,000 and 32,000. A non-radioactive immunoprecipitation technique was employed, which uses chemiluminescence detection of biotin-labelled red cell proteins. The protein of M(r) 32,000, which carries CD99, was identified by this method and the red cell quantitative polymorphism of CD99 was demonstrated. When the Xga protein was precipitated from biotin-labelled red cells, a protein of M(r) 32,000 was coprecipitated. This suggests that the proteins carrying the Xga antigen and CD99 are associated in the membrane.