Several transmembrane molecules are cleaved at juxtamembrane extracellular sites leading to shedding of ectodomains. We analysed shedding of members of the Vps10p-D (Vps10p domain; where Vps is vacuolar protein sorting) family of neuronal type-I receptors with partially overlapping functions, and additional proteolytic events initiated by the shedding. When transfected into CHO (Chinese-hamster ovary) cells (CHO-K1), sorCS1a–sorCS1c isoforms were shed at high rates (∼0.61%·min−1) that were increased approx. 3-fold upon stimulation with phorbol ester. sorCS1c identified in the cultured neuroblastoma cell line SH-SY5Y was shed similarly. In CHO-K1 transfectants, constitutive and stimulated shedding of sorCS3 also occurred at high rates (0.29% and 1.03%·min−1). By comparison, constitutive and stimulated shedding of sorLA occurred at somewhat lower rates (0.07% and 0.48%·min−1), whereas sorCS2 and sortilin were shed at very low rates even when stimulated (∼0.01%·min−1). Except for sorCS2, shedding of the receptors was dramatically reduced in mutant CHO cells (CHO-M2) devoid of active TACE (tumour necrosis factor α-converting enzyme), demonstrating that this enzyme accounts for most sheddase activity. The release of sorCS1 and sorLA ectodomains initiated rapid cleavage of the membrane-tethered C-terminal stubs that accumulated only in the presence of γ-secretase inhibitors. Purified shed sorLA bound several ligands similarly to the entire luminal domain of the receptor, including PDGF-BB (platelet-derived growth factor-BB) and amyloid-β precursor protein. In addition, PDGF-BB also bound to the luminal domains of sorCS1 and sorCS3. The results suggest that ectodomains shed from a subset of Vps10p-D receptors can function as carrier proteins.

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