What type of proteins do signal sequences primarily target?

Signal sequences primarily target secretory proteins because these sequences function as molecular addresses that guide the nascent polypeptide chains to the endoplasmic reticulum (ER) for translation and subsequent processing. In eukaryotic cells, proteins that are meant to be secreted outside the cell or delivered to organelles like lysosomes typically have signal peptides that direct their entry into the secretory pathway.

As the ribosome synthesizes these proteins, the signal sequence is recognized by the signal recognition particle (SRP), which halts translation temporarily and directs the ribosome to the ER membrane. Upon arrival at the ER, the growing polypeptide is threaded through the ER membrane, where it can undergo folding and modifications required for its function.

This mechanism is crucial for maintaining proper cellular function and ensuring that proteins reach their intended destinations to carry out their roles efficiently within or outside the cell.