The MHR73 MoAb recognizes RP105. (A) Cell surface proteins on M12HRP cells were biotinylated. After extraction with a lysis buffer, immunoprecipitation was conducted with the MHR73 MoAb. Precipitated proteins were subjected to SDS-PAGE (12% acrylamide, reduced condition) and blotted onto a nitrocellulose membrane. Blotted proteins were detected with either avidin horseradish peroxides (for cell surface molecules: lane 1) or an anti-flag MoAb followed by goat antimouse IgG-horseradish peroxidase (for human RP105 that bears the flag epitope at the carboxy-terminus: lane 2). Signals of about 50 and 28 kD are observed in lane 2. They correspond to the heavy chain and the light chain of the MHR73 MoAb, because they were detected with goat antimouse IgG-horseradish peroxidase alone (data not shown). (B) Indicated cell lines were subject to cell surface biotinylation. Cell surface molecules were then precipitated with either the MHR73 MoAb (lanes 1, 2, and 4) or the anti-flag MoAb (lane 3). Precipitated molecules were resolved on SDS-PAGE, blotted on an nitrocellulose membrane, and detected with avidin-horseradish peroxidase. The MD-1 signal was very faint in Daudi or Ramos cells in this experiment. However, we confirmed additional signals of 25 and 22 kD in other experiments (data not shown).