In the case of prokaryotes, this essentially means the cell membrane, but for eukaryotes, there are specific signal peptides that can direct the protein to the nucleus, to the mitochondria, to the endoplasmic reticulum, and other intracellular organelles. Signal peptides direct the protein from the cytoplasm into a particular cellular compartment. 11.2: Protein Trafficking A major class of cleaved peptide sequences is signal peptides.Some of the pre-cleavage polypeptides are immediately cleaved, while others are stored as inactive precursors to form a pool of enzymes (or other kinds of proteins) that can be activated very quickly, on a timescale of seconds to minutes, as compared to having to go through transcription and translation, or even just translation.
11.1: Proteolytic Cleavage The most common modification is proteolytic cleavage.