The cytoplasmic/transmembrane domain of dipeptidyl peptidase IV, a type IIglycoprotein, contains an apical targeting signal that does not specifically interact with lipid rafts

We investigated the signals involved in the apical targeting of dipeptidyl peptidase IV (DPP IV/CD26), an archetypal type II transmembrane glycoprotei n. A secretory construct, corresponding to the DPP IV ectodomain, was first stably expressed in both the enterocytic-like cell line Caco-2 and the epi thelial kidney MDCK cells. Most of the secretory form of the protein was de livered apically in MDCK cells, whereas secretion was 60% basolateral in Ca co-2 cells, indicating that DPP IV ectodomain targeting is cell-type-depend ent. A chimera (CTM-GFP) containing only the cytoplasmic and transmembrane domains of mouse DPP IV plus the green fluorescent protein was then studied . In both cell lines, this chimera was preferentially expressed at the apic al membrane. By contrast, a secretory form of GFP was randomly secreted, in dicating that GFP by itself does not contain cryptic targeting information. Comparison of the sequence of the transmembrane domain of DPP IV and sever al other apically targeted proteins does not show any consensus, suggesting that the apical targeting signal may be conformational. Neither the DPP IV nor the CTM-GFP chimera was enriched in lipid rafts. Together these result s indicate that, besides the well-known raft-dependent apical targeting pat hway, the fate of the CTM domain of DPP IV may reveal a new raft-independen t apical pathway. (C) 2001 Academic Press.