The cytoplasmic domain of CR1 contains 2 PDZ motifs and binds PDZ domains 2, 3, and 5 of FAP-1. (A) Cytoplasmic domain of CR1 with its 2 PDZ motifs underlined. (B) A peptide corresponding to the CR1 cytoplasmic domain (CR1-tail peptide) was used to probe 2 PDZ arrays, which together contained all 5 PDZ domains of FAP-1. CR1-tail peptide (100 ng/mL) bound to FAP-1 PDZd2 and 3 (left blot, duplicate 10 ng/spot) and PDZd5 (right blot, 8 or 40 ng/spot). Bound CR1-tail peptide was detected by rabbit anti–Cyto-CR1 (BW60), followed by HRP-conjugated secondary antibody. (C) Surface plasmon resonance sensorgram confirms binding of CR1-tail peptide to immobilized PDZd5 of FAP-1. GST-PDZd5 was immobilized via anti-GST antibody to CM5 sensor chip and probed with CR1-tail peptide (trace b). On the control channel, GST alone was immobilized to assess the background binding of CR1-tail peptide (trace a). Equal volumes of the same concentration of the CR1 peptide were used for both channels. Sensorgrams are representative of 2 experiments. RU indicates resonance units; time is expressed in seconds.