What is removed by a signal peptidase during protein sorting?

The removal of the ER signal sequence by a signal peptidase is a critical step in protein sorting within the cell. When proteins are synthesized in the ribosomes, some of them possess a specific sequence of amino acids at their N-terminus known as the ER signal sequence. This sequence directs the nascent protein to the endoplasmic reticulum (ER), where it is targeted for further processing and eventual transport to its final destination.

Once the protein is translocated into the ER lumen, the signal peptidase cleaves the signal sequence from the precursor protein. This cleavage is essential as it allows the protein to undergo proper folding and maturation without the signal sequence, which is no longer needed after the protein has entered the ER.

This process not only ensures that proteins are correctly sorted and modified as they traverse the secretory pathway but also plays a pivotal role in maintaining the functional integrity of the proteins after their synthesis and transportation. Therefore, the removal of the ER signal sequence by a signal peptidase is a fundamental aspect of protein sorting and maturation.