Final answer:
The signal peptide in water-soluble ER proteins is cleaved by signal peptidase, an enzyme that acts once the protein is inside the endoplasmic reticulum. This step is essential for proper protein localization and function.
Step-by-step explanation:
In water-soluble ER proteins, the signal peptide is cleaved by an enzyme called a signal peptidase. When these proteins are synthesized by ribosomes, they have a signal sequence at their leading end which allows them to be directed into the endoplasmic reticulum (ER). Once inside the ER, the signal peptide is recognized and cleaved off the nascent protein by the signal peptidase to allow the protein to function properly within the cell or be sent to its correct destination.
This process is crucial for the proper localization and function of proteins within different compartments of the cell, and it is a fundamental aspect of cellular biology and protein biogenesis. The mechanism by which these proteins are directed to and translocated across the ER membrane involves a complex set of molecular interactions and machinery, including the signal recognition particle (SRP), SRP receptor, and the translocon complex.