Retention of membrane proteins by the endoplasmic reticulum.

Abstract

We have used a monoclonal antibody specific for a hydrocarbon-induced cytochrome P450 to localize, by electron microscopy, the epitope-specific cytochrome P450. The cytochrome was found in the rough and smooth endoplasmic reticulum (ER) and the nuclear envelope of hepatocytes. Significant quantities of cytochrome P450 were not found in Golgi stacks. We also could not find any evidence of Golgi-associated processing of the Asn-linked oligosaccharide chains of two well-characterized ER membrane glycoprotein enzymes (glucosidase II and hexose-6-phosphate dehydrogenase), or of the oligosaccharides attached to the bulk of the glycoproteins of the ER membrane. We conclude that these ER membrane proteins are efficiently retained during a process of highly selective export from this organelle.Evidence from subcellular fractionation (1-5; and reviewed in reference 6) suggests that membrane proteins (enzyme markers) that are most concentrated in the endoplasmic reticulum (ER) l membranes are also found at high concentrations in the Golgi complex, a highly compartmentalized organelle (6-12).To examine this issue further, we have determined the intraceUular localization of a major ER membrane protein, cytochrome P450 (13), by electron microscope immunocytochemistry. We have also studied the structures of the oligosaccharide chains of two particular ER membrane glycoproteins (glucosidase II [14-16] and hexose-6-phosphate dehydrogenase [H6PDH; 17]) as well as those of a broad spectrum of membrane glycoproteins prepared from ER fractions of rat liver to seek evidence of Golgi-associated oligosaccharide processing of these glycoproteins.

Golgi dispersal during microtubule disruption: regeneration of Golgi stacks at peripheral endoplasmic reticulum exit sites.

“…Regeneration of Golgi stacks over time in these peripheral sites would reestablish secretory flow from the ER to the Golgi and result in "Golgi fragmentation." the ER (Brands et al, 1985;Persson et al, 1992). Nevertheless, it is possible that Golgi enzymes spend only short periods of time passing through the ER, and therefore are difficult to detect there.…”

The trans Golgi Network: Sorting at the Exit Site of the Golgi Complex

“…Productivity growth has decreased in almost all of the major sectors of the economy. Within the major sectors, the Bureau of Labor Statistics (BLS) looked at specific industry-level performance and found that three-quarters of the industries in their sample had declines in productivity growth (2).…”

The highly conserved DAD1 protein involved in apoptosis is required for N‐linked glycosylation

“…The constructed T7-DAD1 cDNA complemented the tsBN7 cells (data not shown), indicating that the T7 fused DAD1 protein has a normal DAD1 function. As a marker of the ER, we used P450 (M1), which is a well known membrane protein localized in the ER (Brands et al 1985;Yamamoto et al 1985;Matsumoto et al 1986). T7-DAD1 cDNA was introduced into the mammalian expression vector pcDEB.…”