Biogenesis of microsomal membrane glycoproteins in rat liver. I. Presence of glycoproteins in microsomes and cytosol.

Abstract

The glycoproteins of microsomes and cytosol were studied. Various washing procedures did not release the proteins from the microsomes, and immunological tests demonstrated that the sialoproteins are not serum components. Low concentrations of deoxycholate and incubation in 0.25 M sucrose solution liberated a small amount of microsomal sialoprotein and this fraction exhibited a high degree of labeling of protein-bound N-acetylneuraminic acid. A part of the glycoprotein fraction could not be solubilized, even with a high concentration of the detergent. Thoroughly perfused rat liver contained sialoproteins in the particle-free supernate. The level of sialoprotein present could not be due to contamination with serum or broken organelles. The high in vivo incorporation of [SH]glucosamine into protein-bound sialic acid of Golgi membranes and cytosol was paralleled by a delayed and lesser rate of incorporation into the rough and smooth microsomal membranes. This incorporation pattern suggests the possibility that the glycoproteins of cytosol and Golgi may later be incorporated into the membrane of the endoplasmic reticulum.

Analytical study of microsomes and isolated subcellular membranes from rat liver. VII. Distribution of protein-bound sialic acid.

“…Our conclusion is consistent with the demonstration made by using iodinated lectins that, in the liver endoplasmic reticulum, membrane glycoproteins do not have the terminal trisaccharide : N-acetylglucosamine-galactose-sialic acid (32) . It is at variance with the reports of others describing sialoglycoprotein as a genuine constituent of the endoplasmic reticulum (5,7,12,19,23,28) . However, this latter conclusion is based upon the biochemical properties of microsomal fractions in which the vesicles derived from the endoplasmic reticulum were probably contaminated by sialoglycoprotein-rich components .…”

“…and 7 which refer to original and to digitonin-treated plasma membranes, respectively, and give both the biochemical characteristics of the preparations used and the density distribution of some constituents . The two preparations were similar with regard to the relative specific activities of enzymes (14-15 for alkaline phosphodiesterase I) and relative specific content in sialoglycoprotein (7)(8). The difference in yield between sialoglycoprotein and alkaline phosphodiesterase I is not a consequence of digitonin treatment, since the same difference was found in untreated fractions.…”

“…Several publications have reported that protein-bound sialic acid (sialoglycoprotein) is not confmed to the pericellular membrane but also occurs within the cell, in the nuclear membrane (21,22,26), mitochondria (13,14,35), lysosomal membrane (20), Golgi complex (5,23), endoplasmic reticulum (5,7,19,24,28), and cytosol (7) (see also reference 41 for a review) . However, in most of these publications it has not been unequivocally established that sialoglycoprotein belongs to the main cell component of the preparation analyzed rather than to a sialoglycoprotein-rich contaminant, for instance the plasma membrane .…”

The biosynthesis of cytochrome P‐450 in vitro

“…It has been suggested that some membrane proteins are inserted directly into the membrane in the course of biosynthesis [3-61. However, recent work suggests that the pathway of biosynthesis of membrane glycoproteins is similar to that of exported proteins and involves intracisternal passage to the Golgi apparatus [7,8]. Since highly purified cytochrome P-450 contains carbohydrate [9] , and is probably a glycoprotein, we have investigated the possibility that this membrane protein passes into the intravesicular space in the course of biosynthesis in vitro in vesicles derived from rough endoplasmic reticulum.…”

GLYCOPROTEINS AND GLYCOLIPIDS OF SUBCELLULAR FRACTIONS FROM BOVINE RETINA. INCORPORATION OF MANNOSE AND SIALIC ACID1

“…We have not attempted to characterize the different membrane fractions obtained after centrifugation of the ROS free membranes in a sucrose density gradient. It has been claimed that in liver, sialyl transferase is a marker for Golgi derived membranes (AUTUORI et al, 1975). In the same organ, on the other hand, the activity for mannosylation of proteins from either GDP-mannose, dolichol-monosphosphate mannose or dolichol-pyrophosphate oligosaccharidemannose was detected in microsomal membranes (BEHRENS et al, 1973;DE TOMAS et al, 1975).…”