A monoclonal antibody, R1, and a polyclonal serum, S10, recognize the same molecules: A novel use of DNA transfectants

The monoclonal antibody R1 defines the product of the MIC5 locus which has been localized to the long arm of the human X chromosome in the same region as the fragile site FRAXA. DNA transfectants, selected by R1 on the fluorescence-activated cell sorter (FACS), have been used to demonstrate that a previously described polyclonal antiserum, S10 (Buck & Bodmer, 1976), recognizes the same cell surface molecule as R1. Immunoprecipitation from metabolically and surface-labelled cells has shown that this molecule is a Mr 200 K phosphoprotein which is synthesized as a Mr 180 K precursor and subsequently modified to a Mr 195 K extracellular form. A Mr 150 K molecule is coprecipitated with the Mr 200 K phosphoprotein, but is only detected in lysates of surface-labelled cells, raising the possibility that the 200 K molecule is a cell surface receptor and the 150 K molecule a ligand.