The Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is a strikingly complex structure within the cell and has three morphologically distinct regions: 

1. Sheets of the nuclear envelope
2. A network of interconnected peripheral ER tubules
3. Peripheral ER sheets

Structure of ER, EMBO reports (2010) 11, 515–521

 All three regions exist within the continuous membrane bilayer and corelate with specialized ER functions. They can be easily detected by flourescence microscopy.

 Structure of RER and SER

The peripheral ER extends throughout the cell cytoplasm and its lumen is continuous with that of the nuclear envelope. Both constitute more than 10% of the total cell volume.

ER sheets are flat, extended surfaces with little membrane curvature that are often characterized by the presence of multiple membrane-bound ribosomes reffered to as rough ER (RER). Thus they tend to be granular in appearance. An example of RER sheet is the nuclear envelope thats considered to be a sheet because of its large size.

ER tubules are long, cylindrical with a lot of membrane curvature and intersect to form an intricate network throughout the cell cytoplasm. Composed primarily of ribosome-free smooth ER (SER), they appear to be more dilated and convoluted in general.

The thickness of sheets and diameter of tubules varies depending on the type of cell they are found in but is typically considered to be around 60-100 nm. This suggests that both regions are actively shaped.

Functions of RER and SER

Since protein translocation is an essential process in all eukaryotic cells, they all contain RER. The membrane-bound ribosomes perform all functions associated with the biosynthesis of proteins that are either secreted or are membrane-bound. This even includes their modification(s) as well as proper folding.

One type of ER that is found in almost all cells is the transitional ER. It is rich in proteins involved in the packaging of other proteins for transport from the ER to the Golgi apparatus.

On the other hand, SER is abundant in certain cell types only. Some of them are:

• steroid-producing cells - functions as a site for many of the step in the synthesis of steroids
• liver cells - important for detoxification of hydrophobic substances
• neurons - assumed to be required for calcium handling
• muscle - called sarcoplasmic reticulum (SR) and is primarily involved in calcium release and uptake in the muscle

But why is it that ribosomes are concentrated (almost exclusively) in the RER and completely excluded from SER rather than existing there at lower concentrations?

Well, researchers have proposed that concentrating the ribosomes in one part of the membrane results in efficient performance on part of the proteins that are associated with the translocation and modification of newly synthesized proteins.

But exactly how the two maintain their different protein compositions remains unknown :-S

Association with other organelles

Some regions of the ER are closely associated with other organelles:

• Golgi apparatus
• mitochondria
• peroxisomes
• vacuoles
• chloroplasts
• plasma membrane

Why so?

Two reasons:

1. Direct lipid and sterol transfer - Since lipids required by all the organelles are synthesized in the ER, it only makes sense that direct transfer of lipids and sterols to the membranes be carried out since not all organelles are associated with the ER through means of transport vesicles (for example, mitochondria)
2. Calcium signaling -  calcium release is facilitated upon membrane depolarization by contact between ER and plasma membrane (for example, muscle cells)

ER Dynamics

During cell division, the ER is subject to rearrangements in order to be properly redistributed to the daughter cells. As the nuclear envelope disassembles, its associated proteins are released into peripheral ER. The peripheral ER in turn undergoes changes in morphology.

What kinds of morphological changes?

It becomes a highly intricate tubular structure with absence of sheets. It has been proposed that membrane fusion plays a likely role in the change in ER morphology - a process that is GTP dependent and involves proteins called atlastins.

During interphase, however, the peripheral ER is a dynamic network consisting of cisternal sheets and linear tubules.