HEPARIN-BINDING EGF-LIKE GROWTH-FACTOR, WHICH ACTS AS THE DIPHTHERIA-TOXIN RECEPTOR, FORMS A COMPLEX WITH MEMBRANE-PROTEIN DRAP27 CD9, WHICH UP-REGULATES FUNCTIONAL RECEPTORS AND DIPHTHERIA-TOXIN SENSITIVITY/

DRAP27, the monkey homolog of human CD9 antigen (DRAP27/CD9) and dipht heria toxin receptor (DTR) were expressed in mouse L cells. L cells tr ansfected transiently with both DRAP27/CD9 and DTR cDNA bound similar to 10 times more diphtheria toxin (DT) than cells transfected with DTR alone. Stable L cell transfectants expressing both DTR and DRAP27/CD9 (LCH-1 cells) had 15 times more cell surface DT-binding sites and wer e 20 times more sensitive to DT than were stable L cell transfectants expressing DTR alone (LH-1 cells). Increased DT-binding and DT sensiti vity were not due to increased DTR transcription or increased cell sur face DTR protein. Co-immunoprecipitation of DRAP27/CD9 with DTR and ch emical cross-linking suggest a tight association of these membrane-bou nd proteins. In addition, the identity of DTR and a growth factor (HB- EGF) was established. Immobilized DT specifically adsorbed HB-EGF prec ursor solubilized from transfected L cells and [I-125]DT bound to immo bilized recombinant HB-EGF. We conclude that DRAP27/CD9 associates tig htly with DTR/HB-EGF and up-regulates the number of functional DTRs an d DT sensitivity, and that HB-EGF is identical to DTR.