What does Endoplasmic reticulum do in a cell? 5 Functions of Rough Endoplasmic Reticulum (RER)

Which is the largest organelle inside the cell?

We often think of nucleus in animal cell and vacuole in plant cell. As per the latest studies, entire endoplasmic reticulum enclosed by a continuous membrane is the largest organelle of most eukaryotic cells. Its membrane may account for about half of all cell membranes, and the space enclosed by the ER (the lumen, or cisternal space) may represent about 10% of the total cell volume.

For better understanding, you can watch our video on ER here.

What is Endoplasmic Reticulum (ER)?

• ER is an interconnected network of flattened, sacs and tubules; that is flattened sacs in Rough Endoplasmic Reticulum (RER) and tubules in Smooth Endoplasmic Reticulum (SER), forming a continuous sheet enclosing a single internal space called the ER lumen or the ER cisternal space.
• ER consists of sheets, tubules, and the nuclear envelope as the outer nuclear membrane is continuous with the rough endoplasmic reticulum and shares a common lumen with RER.
• ER is a network of tubes that produce, modifies and transports proteins, lipids and other molecules throughout the cell. It also serves as the transportation channel in an eukaryotic cell.

Types of ER

There are two distinct types of ER that perform different functions within the cell.

•    The rough ER, which is covered by ribosomes on its outer surface, functions in protein synthesis and processing.
•    The smooth ER is not associated with ribosomes and is involved in lipid metabolism; that is why called smooth.

Functions of Rough Endoplasmic Reticulum (RER)

1.  1. Protein Synthesis

·      One of the major functions of the ER is to serve as a site for protein synthesis for secreted and integral membrane proteins, as well as a subpopulation of cytosolic proteins as per the recent studies.

2. Protein folding using chaperones and other folding enzymes.

Binding immunoglobulin protein (BiP) is an Hsp70 chaperone located in the lumen of the ER. BiP assists in the folding of newly synthesized polypeptides by binding to exposed hydrophobic side chains and subsequently coordinating the formation of their correct tertiary and quaternary structure. Its association with nascent polypeptides is stabilized by the high concentrations of Ca2+ in the ER lumen and likely involves Ca2+ binding.

3. Protein processing or modification like di-sulphide bond formation and N-linked glycosylation

• Disulfide bond formation in proteins occurs exclusively between cysteine sidechains via the oxidation of thiol groups that is required for protein’s tertiary and quaternary structure. 
• The cytosol is an unfavorable environment for disulfide formation because it contains robust NADPH-dependent reducing pathways to maintain proteins in a reduced form. 
• But ER contains distinct enzymatic pathways involving the protein disulfide isomerase (PDI) family of proteins for di-sulphide bonding and the ER lumen is more oxidizing than that of the cytosol favoring di-sulphide bond formation.

4. Initial Glycosylation

Glycosylation or addition of sugar to proteins is an important modification to eukaryotic proteins because the added sugar residues are often used as molecular flags or recognition signals to other cells than comes in contact with them.

N-linked glycosylation begins in the endoplasmic reticulum, but O-linked glycosylation does not occur until the polypeptide has been transported into the Golgi apparatus.

5. Quality control that means misfolded proteins are degraded in ER called as ER associated degradation (ERAD).

Now let's understand the functions of SER.