Question

Predict which organelles would be likely to have received their proteins via posttranslational import

Remember to consider that posttranslational import is an alternative mechanism for import into the ER lumen.

List the types of proteins that are synthesized by ribosomes bound to the ER membrane discussing where you would expect to find these proteins.

Describe the process of cotranslational import into the ER lumen, identifying the main components involved in this process and discussing the role of the ER signal sequence

Answer 1

Protein synthesis, or translation, is known to take place on the ribosomes present in the cytosol of a cell. However, besides these free cytosolic ribosomes, certain ribosomes can also found bound to the ER (Endoplasmic Reticulum) membrane of an eukaryotic cell (there is no ER in prokaryotes). The presence of ribosomes on the ER membrane render a rough texture to the ER surface, which is why this category of endoplasmic reticulum (with ribosomes attached on it) is known ar RER or Rough Endoplasmic Reticulum. Besides protein synthesis, the RER is also responsible for post-translational modification of proteins before being transported to their target organelles. The modifications which polypeptides undergo in the ER membrane and ER lumen include formation of disulfide bonds, glycosylation (addition of carbohydrate groups) of amino acids, protein folding, assembling into multimeric proteins, and also specific proteolytic cleavages to yield the final product. The destination of these post-translationally modified proteins are :

• The cell-membrane
• The extracellular space (which is not an organelle, but receives proteins after posttranslational modification in the ER lumen)
• Golgi apparatus
• Lysosomes
• Some proteins, post modification, are retained in the ER lumen.

The type of proteins that are synthesized on the RER along with the target organelles where these proteins end up, include :

• Secretory proteins, which get secreted into the extracellular space either without any tag, or paired with basic amino acids forming the signal peptide, or within clathrin-coated vesicles.
• Integral membrane proteins, which reside in the cell membrane as a transmembrane protein.
• Lysosomal enzymes, which sometimes get tagged with a marker in the Golgi apparatus (like mannose-6-phosphate), before being transported to the lysosome.
• Proteins of the Golgi bodies, which end up in the Golgi apparatus.
• Proteins of the RER, which remain embedded in the RER membrane.