BETA-1,4-N-ACETYLGALACTOSAMINYLTRANSFERASE (G(M2) SYNTHASE) IS RELEASED FROM GOLGI MEMBRANES AS A NEURAMINIDASE-SENSITIVE, DISULFIDE-BONDEDDIMER BY A CATHEPSIN D-LIKE PROTEASE

Many Golgi membrane-bound glycosyltransferases are released from cells in a soluble form. To characterize this release process, we stably tr ansfected Chinese hamster ovary cells with three myc epitope-tagged fo rms of cloned beta 1,4-N-acetylgalactosaminyltransferase (GalNAcT); tw o of these forms resided in the Golgi, while the third was retained in the ER, GalNAcT was released into the culture medium from cells trans fected with the Golgi forms but not with the ER form of the enzyme. Th e medium from cells transfected with the Golgi forms contained disulfi de-bonded dimers of GalNAcT, which carried neuraminidase sensitive, co mplex N-linked carbohydrate chains. This soluble species represented t he major degradation product of cellular GalNAcT, which turned over wi th a half-time of about 1.7 h. The soluble species consisted of a mixt ure of truncated GalNAcT molecules, the major form of which was produc ed by cleavage near the boundary between the transmembrane and lumenal domains between Leu-23 and Tyr-24. This cleavage site fits the sequen ce pattern for sites cleaved by cathepsin D (van Noort, J.M., and van der Drift, A.C.M. (1989) J. Biol. Chem. 264, 14159-14164), These findi ngs suggest that GalNAcT is converted from a membrane-bound to a solub le form as a result of cleavage by a cathepsin D-like protease in a co mpartment late in the Golgi secretory pathway.