Protein-protein interaction in a three-domain conformation. The presence of constructs α1-trc and Myc-iDΔ62-TM4 is indicated by a plus sign. Furthermore, either the TM3-4 loop construct without an N-terminal Myc epitope or the Myc-TM3-4 loop harboring the Myc epitope was present. A, input of α1-trc, Myc-iDΔ62-TM4, and the Myc-TM3-4 loop detected with MAb4a or α-Myc (9E10) antibody. The lower protein band in the input shows Myc-iDΔ62-TM4; the upper band refers to the Myc-TM3-4 loop. Note the small difference in molecular mass between the two polypeptides (Myc-iDΔ62-TM4, 13 kDa; Myc-TM3-4 loop, 16 kDa). B, co-immunoprecipitation (IP) of the three-domain receptor. Following immunoprecipitation, the TM3-4 loop was never detectable. The bottom panels show Myc-iDΔ62-TM4 detected with an α-Myc antibody. The α1-trc was stained concurrently using the pan-α antibody MAb4a.