Final answer:
Proteins can be targeted to specific subcellular compartments through a process called protein targeting. Signal sequences, which act like train tickets, direct proteins to their intended destination. Other mechanisms, such as receptor proteins and cytoskeletal elements, also contribute to protein targeting.
Step-by-step explanation:
Proteins can be targeted to specific subcellular compartments through a process called protein targeting. This process involves the presence of signal sequences, which act like train tickets to direct proteins to their intended destination. Signal sequences are short tails of amino acids that are recognized by cellular factors and aid in transporting proteins to their correct compartments. For example, a specific sequence at the amino terminus can target a protein to the mitochondria or chloroplasts in plants.
Proteins are initially synthesized in the cytoplasm, and those destined for organelles contain oligopeptide traffic signals that guide them to the appropriate destination. In the case of secretory and membrane proteins, their glycosylation begins in the rough endoplasmic reticulum (RER). Transport vesicles bud off from the RER and carry packaged and membrane proteins to the cis vesicles of the Golgi apparatus. Sortings proteins then associate with different packaged proteins in the trans Golgi vesicles, and the packaged proteins sort to vesicles that move to their final destinations.
In addition to signal sequences, other mechanisms contribute to protein targeting. These include receptor proteins in the plasma membrane that have specific binding affinity for certain substances, as well as cytoskeletal elements that create microcompartments within membranes.