Subcellular site of glycoprotein synthesis in liver

The Secretory Pathways of Rat Serum Glycoproteins and Albumin

“…Although there is some evidence to the contrary, the first sugar, N-acetylglucosamine, is thought to be added to the incompleted polypeptide chain while it is still attached to the polysomes . Mannose, the other sugar which is closest to the protein, has also been shown to be incorporated mostly in the rough ER ; and the sugars situated at the terminal part of the chain are added in the smooth ER and the Golgi apparatus (9)(10)(11)(12)(13)(14)(15)(16) . Although albumin is not a glycoprotein, it still travels the same secretory route as glycoproteins in that it can be located in the rough and smooth ER and in the Golgi apparatus (29)(30)(31)(32)(33) .…”

“…This agrees with the work done with rat liver, mouse kidney, and myeloma cells which also indicates that glycoproteins are synthesized by membrane-attached polysomes (12,25,26) . The majority of evidence now favors the view that the first sugar moiety, N-acetylglucosamine, of hepatic glycoproteins is incorporated into the incompleted polypeptide chain (9)(10)(11)(12)(13)(14)(15)(16) . The site of incorporation of mannose has not, however, been well elucidated .…”

“…The washed microsomes were then subfractionated, as described under Methods, into a ribosomal fraction, a membrane fraction, and the cisternal fraction . The cisternal fraction was diluted five times with water and centrifuged at 105,000 g for 15 hr to obtain a pellet and soluble fraction . The last two columns show the distribution of radioactivity in these fractions .…”

“…It has been suggested that Nproteins pass into the endoplasmic reticulum acetylglucosamine is incorporated while the (ER) (I-8) . Serum proteins are to a large ex-nascent protein is on the polysomes but that tent glycoproteins and they are thought to be mannose and galactose are put on after the newly synthesized by the sequential addition of carbo-formed protein has left the polysomes and is on the intracellular path which leads to secretion (9)(10)(11)(12)(13)(14)(15)(16) . Since N-acetylglucosamine and mannose are thought to be added to the glycoprotein at an early stage of glycoprotein biosynthesis, either at the polysomal or at the rough endoplasmic reticulum stage, we have studied the problem of whether these sugars which are closest to the protein core are added to the endogenous nascent protein during the time at which the protein is being vectorially passed to the cisternae of the ER, or whether the process occurs at a subsequent time and at a different location on the rough ER .…”

Golgi Apparatus: Influence on Cell Surfaces

“…Radioautographic techniques have shown the rapid uptake of several labeled sugars by the Golgi apparatus, without any seeming prior passage through the endoplasmic reticulum, and their subsequent transport and secretion (6,25). Biochemical analyses of Golgi apparatus-rich fractions have consistently shown relatively high values of glycosyl (particularly galactosyl) transferases (26).…”