Compartmentation of the rough endoplasmic reticulum

Summary

It has become evident during recent years that a wide variety of proteins are synthesized on membrane-bound polysomes, very many of which are not ultimately secreted from the cell. The majority of proteins appear to go through some form of post-translational modification before the final appearance of an ‘active’ product, and in some cases the polypeptide chain may be modified before the completed protein molecule is released from the ribosome. This then raises the question concerning the possibility of the organization of the rough endoplasmic reticulum into individual domains, or compartments, each of which may have the responsibility of performing definite and well defined functions. During recent years the behaviour of two subfractions of the rough endoplasmic reticulum in a variety of cell types and under a variety of conditions has been studied in order to gain insight into a possible compartmentation of this organelle.

Throughout the studies disruption of cells has been performed by nitrogen cavitation. This technique was chosen in order to provide conditions of homogenization which were extremely reproducible since shearing forces, mechanical damage and the effects of local heating were eliminated.

Endoplasmic reticulum (ER) membranes isolated from the post-mitochondrial supernatant have been separated into subfractions by centrifugation on discontinuous sucrose gradients. By virtue of their high density imparted by the association of ribosomes, rough ER (RER) membranes penetrate 1.4 M sucrose accumulating above either 2.0 M sucrose (light rough -LR membranes) or a cushion of 2.3 M sucrose (heavy rough -HR membranes). Smooth (S) membranes, which are virtually devoid of ribosomes, collect above 1.4 M sucrose.

The HR, LR and S subfractions in MPC-11 cells differ in a number of respects: RNA/protein and RNA/phospholipid ratios, polysome profiles and marker enzymes. When cells were homogenized in buffer containing 25 mM KCl then all three ER subfractions were observed, however, when the buffer contained 100 mM KCl then only the LR and S subfractions were observed in gradients, radioactivity equivalent to that in the HR fraction was not recovered in the other two subfractions. Four times as many light chain immunoglobulin polypeptides were found associated with polysomes of HR membranes compared to LR membranes. The nuclear associated ER (NER), though very active in protein synthesis, was only 20% as active in the synthesis of light chain as the combined LR/HR fraction. Studies with MPC-11 cells showed that the relative amounts of the three ER subfractions were related to the phase of the cell cycle. The major amount of HR membranes occurred in late GI/early S phase, coinciding with the period of maximal light chain immunoglobulin synthesis. The relative amount of S membranes was remarkably constant throughout the cell cycle.

HR, LR and S subfractions have also been observed in L-cell Krebs II ascites cells and HeLa cells, and in $C3H/10T{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ cells after incubation with the tumor promoter TPA. In mouse liver, pancreas, kidney, lung, bone marrow and other normal tissues, however, only the LIZ and S subfractions were identified in gradients. The occurrence of the HR subfraction in rat liver was not promoted by incubation of tissue in culture medium before cell disruption. Neither was the HR fraction detectable in rat pancreas following the in vivo administration of secretin and cholecystokinin/pancreozymin.

When ER membrane profiles were followed in Krebs II ascites cells during step-up conditions promoted by incubation of cells in culture medium after being harvested from the peritoneal cavity, then a time-dependent appearance of ER subfractions was observed: at 3 h appreciable amounts of LR and S membranes were observed while HR membranes were barely evident in gradients, however, at 18 h there were more HR than LR membranes present while the amount of S membranes was unchanged. Though incorporation of ³H-choline into LR and S membranes was greatly stimulated by incubation of Krebs II ascites cells with TPA the HR fraction did not appear at earlier times than in non-treated cells. The TPA-directed stimulation of ³H-choline incorporation into ER subfractions was not prevented when protein synthesis was inhibited by cycloheximide.

Treatment of Krebs II ascites cells with cytochalasin B (5–10 µg/ml) resulted in an increased yield of all three ER subfractions. The major increase was observed in the HR subfraction. At concentrations above 20 µg/ml however, HR membranes were converted into LR membranes as judged by sedimentation characteristics. The observations suggest a loss of polysomes from HR membranes promoted by cytochalasin B at high concentration in vivo. High concentrations of cytochalasin B added in vitro to isolated HR membranes did not result in a conversion to the LR type. The results indicate that the actual yield of the respective ER subfractions after cell disruption is dependent on the degree of direct and/or indirect interaction between the different ER types and actin containing filaments of the cytoskeleton in the intact cell.

From nutritional studies with L-cells it was evident that the respective amounts of LR and HR membranes differed quite considerably according to the growth conditions. After L-cells were treated with cycloheximide then the HR fraction was no longer observed in gradients while the LR fraction was unaffected. Upon diluting out the inhibitor the HR fraction then re-appeared. These studies together with the fact that RNase treatment of isolated ER membranes modified the sedimentation characteristics of HR (appeared at the LR position) but not of LR membranes, led to the suggestion that polysomes are associated with ER membranes in different ways in the same cell.

After treating Krebs 11 ascites cells for 6 h with TPA a 65 kD protein in the NER showed a 240% increase when compared to untreated cells. The amount of this protein was similar in both untreated and TPA treated cells up to 4 h of incubation. Although the protein was found in HR, LR and S membranes, an increased amount was only observed in the NER. Small changes were also observed in the amounts of several other proteins in the NER in TPA treated cells.

TPA had a dual effect on $C3H/10T{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ cells in that ³H-choline incorporation was stimulated in all subcellular fractions while it simultaneously promoted the solubilization of ³H-choline containing material from prelabeled cells. The release of radioactivity occurred almost exclusively from the NER and in vitro this was dependent on the presence of both Mg²⁺ and Ca²⁺ indicating an enzyme mediated reaction. Data obtained from both Krebs II ascites and $C3H/10T{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ cells indicate that the NER is an early target for TPA action.

Treatment of $C3H/10T{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ cells with TPA or styrene oxide resulted in early increases in ³H-choline label in NER compared to LR/HR membranes. In L-cells chilling followed by incubation at 37° led to a reduction in ³H-choline radioactivity in the NER fraction but an increase in label in LR/HR membranes. Similar results were observed in MPC-11 cells when ³H-glucosamine was utilized. These results suggest that the NER may well represent the site of membrane biogenesis.