Investigation of the Biochemical Relationship between the Blood Group Antigens Xg^a and CD99 (12E7 Antigen) on Red Cells

Abstract

The biochemical relationship between the red cell antigens Xg^a 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 Xg^a-immunoprecipitates with anti-Xg^a has shown that Xg^a antigen is carried on a broad band of apparent molecular weight (M(r)) 24,500-29,500, which consists of a darkly stained component at M(1) 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-l and NaM123 which also recognise CD99, distinguished two bands of M(1) 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(1) 32,000, which carries CD99, was identified by this method and the red cell quantitative polymorphism of CD99 was demonstrated. When the Xg^a protein was precipitated from biotin-labelled red cells, a protein of M(r) 32,000 was coprecipitated. This suggests that the proteins carrying the Xg^a antigen and CD99 are associated in the membrane.