Ribosomes involved in protein synthesis could lie free in the cytoplasm or are bound to membranes of endoplasmic reticulum. Free ribosomes synthesise protein without a signal sequence, and continue synthesis until the polypeptide is completed. Such a protein may assume its three-dimensional conformation with the help of chaperones present in the cytosol.
Some of the important destinations for proteins synthesised on free ribosomes are:
1. The Cytosol:
These are proteins with housekeeping responsibilities, take part in metabolic activities, constituting various enzymes for the metabolic machinery. Include tubulins for generating microtubules, actin for microfilaments. They are released into the cytosol for performing their function.
2. The Nucleus:
Some proteins move from the cytosol into the interior of the nucleus. These include the histones, transcriptional factors and ribosomal proteins. They are targeted to the nucleus by their nuclear localisation sequence, consisting of about 7 to 41 amino acids.
3. Mitochondria:
Even though mitochondria have their own genome, ribosomes, and protein synthesizing machinery, most of the proteins used by mitochondria are encoded by nuclear genes, synthesised in the cytosol on free ribosomes and transported into mitochondria. These proteins possess a characteristic signal sequence which is recognized and bound by a chaperone called mitochondrial stimulation factor (MSF).
MSF targets the protein to a receptor embedded in the outer membrane of the mitochondrion. Other factors and receptors direct the proteins through the inter-membrane space to the inner mitochondrial membrane and the matrix.
4. Chloroplasts:
Like mitochondria, chloroplasts also have their own genome and protein synthesizing machinery. Yet, most of the proteins used in chloroplasts are encoded by the host cell genome. These proteins are synthesised on free cytosolic ribosomes, then transported into the chloroplast.
These proteins have a characteristic transit sequence by which they are recognised, and chaperones are also needed for their transport to their final destination, which could be the chloroplast inner membrane, thylakoid membrane or the stroma.
5. Peroxisomes:
Proteins synthesised for peroxisomes carry a peroxisomal targeting signal (PTS) that binds to a receptor molecule, which internalizes the protein into the peroxisome. The signal sequence then returns to carry another protein load to the peroxisome. Two peroxisome targeting signals have been identified, each having its own specific receptor to take it to the peroxisome.
Proteins Synthesised on Ribosomes Bound to Endoplasmic Reticulum:
Proteins used in membrane-bound organelles such as lysosomes, vesicles and plasma membrane on cell boundary, are synthesised on ribosomes bound to the membranes of endoplasmic reticulum (ER). These are called secretory proteins.
The synthesis of these proteins actually begins on free cytosolic ribosomes that are in no way different from ribosomes bound to ER membranes. During synthesis on free ribosomes, the polypeptide chain is translocated (co-translational translocation) to ER.
The free ribosomes engaged in synthesis of proteins destined for secretion, are targeted to ER by a signal sequence at the amino terminus of the growing polypeptide chain. The signal sequences contain about 20 amino acids, some of which being hydrophobic enable them to cross the ER membrane into the lumen of ER.
The process of co-translational translocation takes place as follows. As the polypeptide chain is emerging from the free cytoplasmic ribosome, the signal sequences are recognised and become bound to a signal recognition particle (SRP). The binding produces a complex consisting of ribosome, SRP and growing polypeptide chain, and further translation is inhibited.
The entire complex is targeted to the ER and binding takes place with the SRP receptor on ER membrane. SRP is released. The signal sequence is inserted into a membrane channel in ER and translocate the polypeptide chain into the lumen of ER.
The proteins then begin their journey in the secretory pathway starting from lumen of ER. They become included in secretory vesicles that are budded off from ER membrane.
They reach the Golgi apparatus, travel through the stacked Golgi membranes, and are then budded out in vesicles. These vesicles target the organelles that are final destinations of the protein, fuse with the organelle membrane and deliver the protein. The proteins undergo their required modifications (for example glycosylation) during their journey in the secretory pathway.