Final answer:
Proteins synthesized on the rough ER include those secreted outside the cell, membrane proteins, and proteins destined for organelles. Proteins made in the ER lumen are post-translationally modified and directed to their final destinations, ensuring proper function and localization, such as peripheral membrane proteins ending up on the outside of the plasma membrane after synthesis in the ER lumen.
Step-by-step explanation:
Types of Proteins Synthesized on the ER and Their Significance
Proteins that are translated on the endoplasmic reticulum (ER), particularly the rough ER (RER), include three main types:
- Proteins destined to be secreted outside the cell.
- Proteins that will be embedded into the plasma membrane of the cell.
- Proteins that will be targeted to various organelles within the cell.
These proteins are synthesized by ribosomes that are attached to the ER. The significance of this localization is that such proteins, from the onset of their synthesis, are segregated from the cytosol and are channeled into the ER lumen, where they begin post-translational modifications, such as core glycosylation, folding, and other crucial modifications.
If a peripheral membrane protein is synthesized within the ER lumen, it will ultimately be oriented on the outside of the plasma membrane once it is expressed on the cell surface. The rough ER also participates in the synthesis of phospholipids for cell membranes, highlighting its role in lipid metabolism as well as protein synthesis and transport.
Such strategic localization and synthesis on the ER are crucial, as proteins can be properly modified and directed to their respective destinations, including secretion outside the cell, integration into the cell membrane, or distribution to other organelles.