Colocalization of beta 1,4galactosyltransferase with mannose 6-phosphate receptor in monensin-induced TGN-derived structures

Previously, we demonstrated that beta1,4-galactosyltransferase (gal-T1) rev ersibly segregates from alpha2,6sialyltransferase (ST6Gal) to swollen vesic les after monensin treatment of the cells. To further explore this phenomen on, we investigated the response to monensin of various Golgi proteins. Wit hin 30 min of monensin treatment, gal-T1 moved from the Golgi apparatus, as defined by localization of giantin, to swollen vesicles whereas ST6Gal, al pha2,3(N)sialyltransferase, mannosidase II, and N-acetylgalactosaminyltrans ferase 2 remained associated with the Golgi apparatus. Stably transfected C HO cells exhibited a similar phenomenon of monensin-induced displacement of recombinant gal-T1 to swollen vesicles while recombinant ST6Gal remained c olocalized with endogenously expressed giantin. Gal-T1 and the cation-insen sitive mannose 6-phosphate receptor colocalized in swollen vesicles as obse rved at both light and electron microscopic levels. When monensin was repla ced by chloroquine, gal-T1 remained arrested in swollen vesicles. Brefeldin A treatment known to cause relocation of Golgi-associated gal-T1 to the en doplasmic reticulum had no effect on gal-T1 trapped in swollen vesicles. Th is evidence suggests that monensin blocks gal-T1 trafficking in post-Golgi structures and argues against swelling of gal-T1-containing trans Golgi cis ternae as previously assumed.