SERT (including a series of double and truncation mutants along its C-terminus) and related transporters were screened for their SEC24 isoform dependence, examined by siRNA-induced SEC24 knock-down in HEK293 cells. The cells were transfected with Sec24 siRNAs and, 48 h later, with transporter plasmids (Lipofectamine, Invitrogen), and the effects were determined by substrate uptake and confocal microscopy. GST-fusion proteins comprising the C-terminus of wild-type and mutant SERTs were used for pull-down experiments carried out with SEC24C and SEC24D. To examine the role of heat shock protein (HSP) 90 in regulating the formation of COPII vesicles, we treated HEK293 cells expressing wild-type SERT and its RI-607,608-AA mutant (putative ER export motif site) with geldanamycin, prior to measuring the effect of HSP90 inhibition on transporter trafficking.