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tone fractions III and IV in cells in which DNA is not undergoing replication, appears to be a general characteristic of higher organisms, plants, and animals.

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The biosynthesis and maturation of the oligosaccharide moieties of the envelope glycoprotein of vesicular stomatitis virus were investigated in virus-infected HeLa and BHK21 cells after pulse labeling with [2-3Hlmannose. Two major forms of the virus glycoprotein were detected by polyacrylamide gel electrophoresis, which appear to correspond to the viral glycoprotein with either "precursor" or "mature" oligosaccharide chains. The precursor chains in both HeLa and BHK21 cells infected with vesicular stomatitis virus obtained after a 30-min pulse were large oligomannose structures containing approximately 7-9 mannose residues as estimated by gel filtration analysis. The size of the oligomannose structures initially transferred to the protein may have been even larger. Mature, virus-size oligosaccharide chains, which could be detected after a 20-to 30-min delay, contained only three mannose residues and, in addition, contained branch structures terminating in sialic acid. A precursor-product relationship of these two forms of oligosaccharide chains was demonstrated by pulse-chase labeling of virus-infected HeLa cells. These studies indicated that the large oligomannosyl core structures initially added to the glycoprotein were being "trimmed" by the removal of mannose residues prior to (and/or during) the addition of the branch chains terminating in sialic acid. Vesicular stomatitis virus (VSV) is an enveloped, RNA-containing virus of the rhabdovirus group which matures by budding through host cell membranes that have been modified by replacement of host proteins with two virus-specified proteins, one of which is a glycoprotein (1). The VSV-infected animal cell is an excellent experimental system for elucidating the molecular and subcellular mechanisms involved in the biosynthesis and maturation of both cell membrane and virus envelope glycoproteins (2-8).The VSV G polypeptide contains two major glycosylation sites with complex oligosaccharides that are linked N-glycosidically to asparagine (9-11). As expected for the biosynthesis of a membrane glycoprotein, mRNA coding for VSV G is localized on membrane-bound polyribosomes in infected cells (12, 13), and newly synthesized G is found predominantly in rough endoplasmic reticulum membranes (14). The sugar residues in the oligomannosyl core [Mann(GlcNAc)2] are apparently added to the G polypeptide in the rough endoplasmic reticulum, whereas the branch sugars (NeuNAc-Gal-GlcNAc) and fucose are apparently added in smooth internal membranes prior to the appearance of the mature glycoprotein at plasma membranes (15). The initial glycosylation event probably occurs by the en bloc transfer of a preformed oligomannosyl core structure from a lipid intermediate (16).We have examined the product of the initial glycosylation of the VSV G glycoprotein and followed its conversion to mature, virus-size oligosaccharide chains in VSV-infected HeLa and BHK21 cells. Our results indicate that a large oligomannosyl core structure [Man>7(GlcNAc)2] is initially added to the glycopro...

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