Secretory proteins migrate from the rough endoplasmic reticulum (ER) to the Golgi complex at different rates. Selective retention of specific proteins to rough-ER membrane constituents could explain this phenomenon. We have permeabilized HepG2 cells with low concentrations of saponin. Release of newly synthesized proteins was studied after brief labelling in the presence of [35S]methionine. The efflux of several secretory proteins was studied at various saponin concentrations; a 2-fold higher saponin concentration was required to release transferrin compared with that required to release albumin and orosomucoid. Glucosidase II, a soluble resident protein of the ER, is released at the same saponin concentration as albumin. Saponin did not destroy the membrane skeleton structure; at the concentrations used, the integral membrane protein G of vesicular-stomatitis virus remained fully associated with the cells.

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