Abstract
The major α1,3fucosyltransferase activity in plasma, liver, and kidney is related to fucosyltransferase VI which is encoded by the FUT6 gene. Here we demonstrate the presence of α1,3fucosyltransferase VI (α3-FucT VI) in the human HepG2 hepatoma cell line by specific activity assays, detection of transcripts, and the use of specific antibodies. First, FucT activity in HepG2 cell lysates was shown to prefer sialyl-N-acetyllactosamine as acceptor substrate indicating expression of α3-FucT VI. RT-PCR analysis further confirmed the exclusive presence of the α3-FucT VI transcripts among the five human α3-FucTs cloned to date. α3-FucT VI was colocalized with β1,4galactosyltransferase I (β4-GalT I) to the Golgi apparatus by dual confocal immunostaining. Pulse/chase analysis of metabolically labeled α3-FucT VI showed maturation of α3-FucT VI from the early 43 kDa form to the mature, endoglycosidase H-resistant form of 47 kDa which was detected after 2 h of chase. α3-FucT VI was released to the medium and accounted for 50% of overall cell-associated and released enzyme activity. Release occurred by proteolytical cleavage which produced a soluble form of 43 kDa. Monensin treatment segregated α3-FucT VI from the Golgi apparatus to swollen peripheral vesicles where it was colocalized with β4-GalT I while α2,6(N)sialyltransferase remained associated with the Golgi apparatus. Both constitutive secretion of α3-FucT VI and its monensin-induced relocation to vesicles analogous to β4-GalT I suggest a similar post-Golgi pathway of both α3-FucT VI and β4-GalT I
Borsig, L