Fig. 4. APP and APLP1 bind to specific contactin family members. (A) AP fusion proteins were tested for binding to contactin-Fc fusion proteins immobilized on Protein A beads. AP activity retained after washing beads is shown. APP bound prominently to contactins 3 and 4. APLP1 bound to contactins 3, 4 and 5. Some non-saturable binding indicating weak or non-specific binding was also seen with contactin 1-Fc (data not shown). (B-F) Saturation curves (insets) were generated by varying the concentration of AP fusion protein. Lines in corresponding Scatchard plots were generated by least squares fitting. K_Ds: contactin 4-APP, 17 nM; contactin 4-APLP1, 28 nM; contactin 3-APP, 22 nM; contactin 3-APLP1, 35 nM; and contactin 5-APLP1, 32 nM.