Figure 1. Identification of gSAP as an imatinib target.

a: A PS1-associated 16 kDa protein is labeled by a photoactivatable imatinib derivative. Left panel: photolysis of ¹²⁵I-G01 with membrane preparations. Middle panel: photolysis of ³H-G01 with intact HEK293 cells. Right panel: PS1-CTF migrated with a slower mobility than the labeled 16 kDa band and was not labeled by G01. Labeling specificity was confirmed by competition with unlabeled imatinib. b: Solubilized endogenous γ-secretase components from HEK293 cells were bound to immobilized biotin-imatinib (left panel). Among the proteins bound to biotin-imatinib, a ~ 16 kDa band was detected by silver staining and was identified as the C-terminal domain of gSAP (right panel, arrow and label “gSAP”). Biotin-coated beads and an inactive biotin-imatinib derivative (see supplementary Fig. 3) served as controls. c: Endogenous gSAP in N2a cells was synthesized as a full length 98 kDa-precursor protein and rapidly processed into a 16 kDa C-terminal fragment. Under steady-state conditions, the predominant cellular form of gSAP was 16 kDa. d: Endogenous gSAP-16K was specifically labeled by ³H-G01 in neuroblastoma cells. e: After gSAP siRNA knockdown in N2a cells, immobilized biotin-imatinib no longer captured PS1.