A) A cartoon of various FLAG-tagged CD74 constructs: the yeast split-ubiquitin (TH) clone, full length CD74, two deletions constructs of CD74, and two splicing variants of mouse CD74. The cytoplasmic and luminal parts, the locations of the FLAG tag, the endosomal-sorting signal in the cytoplasmic domain (LL), the CLIP domain (C), and the trimeric domain (T) are indicated. B) HeLa cells were transfected with pcDNA3 (-) or FLAG-CD74 (+) together with APP, and the total cell lysates (TL) and FLAG immunoprecipitants (IP) were analyzed by Westerm blot. Full length APP, APP CTF (C99 and C83), and CD74 were detected with 22C11, αAPPct, and αFLAG antibodies, respectively. The 33 and 14 kDa N-terminal fragments of CD74 were designated as CD74 NTF33 and NTF14. C) HeLa cells were transfected with FLAG-CD74 and APP, and the total lysates of the transfected cells were immunoprecipitated with either rabbit polyclonal antibody (RP) or αAPPct, and analyzed as in B. D) HeLa cells were transfected with various CD74 constructs and APP. The total lysates and FLAG IPs were analyzed as in B. Mature APP and immature APP (mAPP/imAPP) are indicated. The bands marked with asterisks (*) were attributed to the FLAG antibody used in the immunoprecipitation.