Rab5 binds to a subregion of APP–BP1 within amino acids 443–479. Primary neurons were infected with HSV vectors expressing myc–APP–BP1 and Rab5 or myc–APP–BP1 and Rab7. myc–APP–BP1 was precipitated from the cell lysate with the anti-myc antibody 9E10 or with an irrelevant anti-HA antibody. A, B, The blots were probed with anti-Rab5 (A) or anti-Rab7 (B). Rab5, but not Rab7, coprecipitated with myc–APP–BP1. The lane labeled “whole cell” represents 4% of the lysate used for immunoprecipitation (i.p.). C, To determine the subregion of APP–BP1 to which Rab5 binds, neurons were coinfected with HSV–Rab5 and HSV vectors expressing APP–BP1(Δ401–479), HA–APP–BP1(443–479), or HA–APP–BP1(145–251). Immunoprecipitation was performed with anti-APP–BP1 or anti-HA, and the immunoprecipitate was blotted with an antibody to Rab5. A specific protein band was detected in cells coexpressing APP–BP1 or APP–BP1(443–479) with Rab5 but not in those coexpressing APP–BP1(Δ401–479) or APP–BP1(145–225), suggesting that Rab5 binds to a region within amino acids 443–479 in APP–BP1.