Final answer:
Proteins are directed to the ER by a signal sequence and the rough ER is key in protein synthesis and modification for membrane incorporation or secretion, including the production of phospholipids.
Step-by-step explanation:
All proteins are directed to the ER by a signal sequence that is made up of small hydrophobic amino acids. This signal sequence is recognized by the rough endoplasmic reticulum (RER), where ribosomes usually produce proteins that are destined for the cell membrane or other cellular destinations. The synthesis of membrane-spanning (integral) proteins involves the ribosome translating the protein and directing it to the RER with an N-terminal signal sequence.
However, before an integral membrane protein can pass completely through the membrane, a stop-transfer sequence, which is a hydrophobic domain within the polypeptide chain, traps the protein in the fatty acid interior of the membrane. Such sequences are crucial for the proper insertion and anchoring of integral membrane proteins.
The RER is involved not only in the synthesis of membrane and secretory proteins but also in their modification, such as the addition of carbohydrates and the folding of the polypeptide chains. These modified proteins can be incorporated into the ER membrane or transported to other organelles or even secreted from the cell. Moreover, the RER contributes to the production of phospholipids required for cellular membranes.