What signal is typically involved for proteins intended for secretion?

Proteins intended for secretion typically contain a sequence of amino acids that forms a signal peptide. This signal peptide serves as a molecular address, guiding the nascent protein to the endoplasmic reticulum (ER) during translation. Once the ribosome synthesizes a protein with a signal peptide, this signal is recognized by the signal recognition particle (SRP), which halts translation temporarily and directs the ribosome to the ER membrane.

At the ER, the signal peptide is further processed, allowing the protein to enter the ER lumen or become embedded in the ER membrane, where it can undergo folding, post-translational modifications, and ultimately be packaged into vesicles for secretion. Thus, the presence of a signal peptide is crucial for the proper trafficking of proteins destined for secretion, ensuring they reach their correct cellular destination.

The other options do not apply in this context; termination signals relate to the end of protein synthesis, phosphorylation signals are involved in modifying proteins after translation for their activity or location rather than for their initial targeting, and ribosomal binding sites are related to the initiation of translation, not to the targeting of proteins post-translationally.