Membrane traffic in Dictyostelium discoideum: plasma membrane glycoconjugates internalized and recycled during fluid phase pinocytosis enter the Golgi complex.

Plasma membrane glycoconjugates, enzymatically labelled with [3H]galactose, were used as an autoradiographic membrane marker for a morphometric analysis of membrane traffic during fluid phase pinocytosis in the amoeba, Dictyostelium discoideum. The fraction of grains associated with the plasma membrane decreased exponentially from 99% for cells fixed directly after labelling on the cell surface, to a steady-state value of 45% after about 1 h of pinocytotic activity. The complementary fraction of grains was observed on vacuolar membranes. Only after a lag of about 20 min, a small but significant fraction (3%) of the total grains, was found in the region of the Golgi membranes. During two subsequent doublings of cell number, over a period of 24 h, label was lost into the medium at a constant rate of 1% per h. The cell bound label remained fully membrane bound over the entire period. The beta(1-4) linkage was not noticeably modified, as judged by its susceptibility to hydrolytic release by beta-galactosidase. An analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed an identical labelling pattern for total membrane fractions when prepared directly after labelling or after 24 h of membrane flow.