Proteins are synthesized in two types of location:
- The vast majority of proteins are synthesized by ribosomes in the cytosol.
- A small minority are synthesized by ribosomes within organelles (mitochondria or chloroplasts).
Proteins synthesized in the cytosol can be divided into two general classes with regard to localization: those that are not associated with membranes; and those that are associated with membranes (see 32.5 Membranes and membrane proteins). Figure 8.1 maps the cell in terms of the possible ultimate destinations for a newly synthesized protein and the systems that transport it:
- Cytosolic (or "soluble") proteins are not localized in any particular organelle. They are synthesized in the cytosol, and remain there, where they function as individual catalytic centers, acting on metabolites that are in solution in the cytosol.
- Macromolecular structures may be located at particular sites in the cytoplasm; for example, centrioles are associated with the regions that become the poles of the mitotic spindle.
- Nuclear proteins must be transported from their site of synthesis in the cytosol through the nuclear envelope into the nucleus.
- Most of the proteins in cytoplasmic organelles are synthesized in the cytosol and transported specifically to (and through) the organelle membrane, for example, to the mitochondrion or peroxisome or (in plant cells) to the chloroplast. (Those proteins that are synthesized within the organelle remain within it.)
- The cytoplasm contains a series of membranous bodies, including endoplasmic reticulum (ER), Golgi apparatus, endosomes, and lysosomes. This is sometimes referred to as the "reticuloendothelial system." Proteins that reside within these compartments are inserted into ER membranes, and then are directed to their particular locations by the transport system of the Golgi apparatus. (For an introduction see 32.6 ER and Golgi).
- Proteins that are secreted from the cell are transported to the plasma membrane and then must pass through it to the exterior. They start their synthesis in the same way as proteins associated with the reticuloendothelial system, but pass entirely through the system instead of halting at some particular point within it.