- Protein sorting (targeting) is the direction of different types of proteins for transport into or between specific organelles.
- A signal sequence is a short region of a protein that directs it to the endoplasmic reticulum for co-translational translocation.
- Proteins associate with the ER system only co-translationally.
- The signal sequence of the substrate protein is responsible for membrane association.
Proteins that associate with membranes via N-terminal leaders use a hierarchy of signals to find their final destination. In the case of the reticuloendothelial system, the ultimate location of a protein depends on how it is directed as it transits the endoplasmic reticulum and Golgi apparatus. The leader sequence itself introduces the protein to the membrane; the intrinsic consequence of the interaction is for the protein to pass through the membrane into the compartment on the other side. For a protein to reside within the membrane, a further signal is required to stop passage through the membrane. Other types of signals are required for a protein to be sorted to a particular destination, that is, to remain within the membrane or lumen of some particular compartment. The general process of finding its ultimate destination by transport through successive membrane systems is called protein sorting or targeting, and is discussed in 27 Protein trafficking.
The overall nature of the pathway is summarized in Figure 8.18. The "default pathway" takes a protein through the ER, into the Golgi, and on to the plasma membrane. Proteins that reside in the ER possess a C-terminal tetrapeptide (KDEL, which actually provides a signal for them to return to the ER from the Golgi). The signal that diverts a protein to the lysosome is a covalent modification: the addition of a particular sugar residue. Other signals are required for a protein to become a permanent constituent of the Golgi or the plasma membrane. We discuss direction to these locations in 27 Protein trafficking.
There is a common starting point for proteins that associate with, or pass through, the reticuloendothelial system of membranes. These proteins can associate with the membrane only while they are being synthesized. The ribosomes synthesizing these proteins become associated with the endoplasmic reticulum, enabling the nascent protein to be co-translationally transferred to the membrane. Regions in which ribosomes are associated with the ER are sometimes called the "rough ER," in contrast with the "smooth ER" regions that lack associated polysomes and which have a tubular rather than sheet-like appearance (for review see Palade, 1975). Figure 8.19 shows ribosomes in the act of transferring nascent proteins to ER membranes.
The proteins synthesized at the rough endoplasmic reticulum pass from the ribosome directly to the membrane. Then they are transferred to the Golgi apparatus, and finally are directed to their ultimate destination, such as the lysosome or secretory vesicle or plasma membrane. The process occurs within a membranous environment as the proteins are carried between organelles in small membrane-coated vesicles (see 27 Protein trafficking.)
Co-translational insertion is directed by a signal sequence. Usually this is a cleavable leader sequence of 15-30 N-terminal amino acids. At or close to the N-terminus are several polar residues, and within the leader is a hydrophobic core consisting exclusively or very largely of hydrophobic amino acids (for review see von Heijne, 1985). There is no other conservation of sequence. Figure 8.20 gives an example.
The signal sequence is both necessary and sufficient to sponsor transfer of any attached polypeptide into the target membrane. A signal sequence added to the N-terminus of a globin protein, for example, causes it to be secreted through cellular membranes instead of remaining in the cytosol (Lingappa et al., 1984).
The signal sequence provides the connection that enables the ribosomes to attach to the membrane. There is no intrinsic difference between free ribosomes (synthesizing proteins in the cytosol) and ribosomes that are attached to the ER. A ribosome starts synthesis of a protein without knowing whether the protein will be synthesized in the cytosol or transferred to a membrane. It is the synthesis of a signal sequence that causes the ribosome to associate with a membrane (Blobel and Dobberstein, 1975; for review see Lee and Beckwith, 1986).