- Proteins that are imported into cytoplasmic organelles are synthesized on free ribosomes in the cytosol.
- Proteins that are imported into the ER-Golgi system are synthesized on ribosomes that are associated with the ER.
- Proteins associate with membranes by means of specific amino acid sequences called signal sequences.
- Signal sequences are most often leaders that are located at the N-terminus.
- N-terminal signal sequences are usually cleaved off the protein during the insertion process.
There are two ways for a protein to make its initial contact with a membrane:
- The nascent protein may associate with the translocation apparatus while it is still being synthesized on the ribosome. This called co-translational translocation.
- The protein may be released from a ribosome after translation has been completed. Then the completed protein diffuses to the appropriate membrane and associates with the translocation apparatus. This is called post-translational translocation.
The location of a ribosome depends on whether the protein under synthesis is associating with a membrane co-translationally:
- Co-translational translocation is used for proteins that enter the endoplasmic reticulum. The consequence of this association is that the ribosome is localized to the surface of the endoplasmic reticulum. Because the ribosomes are associated with the ER membranes during synthesis of these proteins, and are therefore found in membrane fractions of the cell, they are sometimes described as "membrane-bound".
- All other ribosomes are located in the cytosol; because they are not associated with any organelle, and fractionate separately from membranes, they are sometimes called "free ribosomes". The free ribosomes synthesize all proteins except those that are translocated co-translationally. The proteins are released into the cytosol when their synthesis is completed. Some of these proteins remain free in the cytosol in quasi-soluble form; others associate with macromolecular cytosolic structures, such as filaments, microtubules, centrioles, etc., or are transported to the nucleus, or associate with membrane-bound organelles by post-translational translocation.
To associate with a membrane (or any other type of structure), a protein requires an appropriate signal, typically a sequence motif that causes it to be recognized by a translocation system (or to be assembled into a macromolecular structure).
Figure 8.5 summarizes some signals used by proteins released from cytosolic ribosomes. Import into the nucleus results from the presence of a variety of rather short sequences within proteins. These "nuclear localization signals" enable the proteins to pass through nuclear pores. One type of signal that determines transport to the peroxisome is a very short C-terminal sequence. Mitochondrial and chloroplast proteins are synthesized on "free" ribosomes; after their release into the cytosol they associate with the organelle membranes by means of N-terminal sequences of ~25 amino acids in length that are recognized by receptors on the organelle envelope.
Proteins that reside within the reticuloendothelial system enter the endoplasmic reticulum while they are being synthesized. The principle of co-translational translocation is summarized in Figure 8.6. An important feature of this system is that the nascent protein is responsible for recognizing the translocation apparatus. This requires the signal for co-translational translocation to be part of the protein that is first synthesized, and, in fact, it is usually located at the N-terminus.
A common feature is found in proteins that use N-terminal sequences to be transported co-translationally to the ER or post-translationally to mitochondria or chloroplasts. The N-terminal sequence comprises a leader that is not part of the mature protein. The protein carrying this leader is called a preprotein, and is a transient precursor to the mature protein. The leader is cleaved from the protein during protein translocation.