Intracellular pathway of a mucin-type membrane glycoprotein in mouse mammary tumor cells

Epiglycanin, a mucin-type glycoprotein, was found by immunoelectron microscopy to be located in cytoplasmic compartments, as well as at the surface of the TA3-Ha mammary carcinoma ascites cell. The glycoprotein was identified by means of gold-labeled secondary antibody bound to a primary anti-epiglycanin monoclonal antibody or by lectins specific for carbohydrate structures in epiglycanin. The primary antibody recognized a glycopeptide component containing a beta-D-(1----3)-D-GalNAc chain attached to a serine or threonine residue. Two routes to the cell surface from epiglycanins's first-recognized location in the trans-Golgi reticulum were suggested. Its presence in vesicles, which fuse with the cell surface, would explain the presence of epiglycanin as an integral membrane protein. Some of these observed vesicles, however, may be endocytotic in character. Epiglycanin was also found in large multivesiculate sacs which were observed on occasion to be open to the extracellular milieu. This finding, as well as the observed fusion of small vesicles from the trans-Golgi network with the sacs, strongly suggested exocytotic migration for the large sacs. Endocytotic migration may also be possible, although incubation of viable cells with gold-labeled antiepiglycanin antibody resulted in minimal uptake within the intracellular sacs, and incubation with [125I]-epiglycanin under metabolic conditions resulted in no detectable uptake of radiolabel by the cells.