Proteins synthesized by bound ribosomes are mainly those destined for the endoplasmic reticulum (ER), lysosomes, Golgi apparatus, cell membranes, and secretion outside the cell. These ribosomes are attached to the cytosolic surface of the rough ER and translate proteins while transferring the growing polypeptide into the ER lumen or inserting it into the ER membrane. Once synthesized, many of these proteins travel through the endomembrane system to reach their final locations.
Bound Ribosomes and the Rough Endoplasmic Reticulum
A bound ribosome is a ribosome attached to the membrane of the rough endoplasmic reticulum. The rough ER appears rough because ribosomes are bound to its cytosolic surface. Although it is continuous with the nuclear envelope, the ribosomes attached to it are part of the broader cytosolic protein-synthesis machinery.
The ER is the entry point for proteins that need to enter an internal membrane compartment or be delivered to the cell exterior. In practice, protein synthesis begins on a ribosome in the cytosol, just as it does for proteins made by free ribosomes. The major difference is the fate of the newly formed polypeptide.
Most bound ribosomes are initially recruited to the ER by a signal-recognition particle, or SRP. Even so, sRP recognizes an ER-targeting signal in the growing polypeptide and directs the ribosome to an SRP receptor on the ER membrane. The ribosome then engages a protein-conducting channel called a translocon Less friction, more output..
Translation continues while the ribosome remains bound to the membrane. As the polypeptide emerges, it can:
- Pass completely into the ER lumen.
- Become embedded in the ER membrane.
- Remain partly in the cytosol and partly within the membrane.
- Be released into the cytosol after its targeting signal is removed.
This process is known as cotranslational translocation because protein synthesis and transfer into the ER occur at the same time.
Major Categories of Proteins Synthesized by Bound Ribosomes
1. Secretory Proteins
Secretory proteins are synthesized by bound ribosomes because they must leave the cytosol and enter the ER before being processed and transported to the cell surface. In the ER, these proteins usually fold, form disulfide bonds, and undergo initial quality control Which is the point..
After leaving the ER, they move into vesicles that carry them to the Golgi apparatus. But there, they are further modified, sorted, and packaged into secretory vesicles. These vesicles eventually fuse with the plasma membrane, releasing the protein outside the cell Which is the point..
Examples include:
- Hormones, such as insulin and many peptide hormones.
- Digestive enzymes, including pancreatic digestive enzymes.
- Antibodies produced by plasma cells.
- Cytokines and interleukins involved in immune communication.
- Extracellular matrix proteins, such as collagen and certain glycoproteins.
- Plasma proteins, including albumin and clotting factors.
Not every molecule released by a cell is a protein. Steroid hormones, for example, are lipids and are not synthesized by ribosomes. That said, many protein hormones and protein-based signaling molecules follow the secretory pathway And that's really what it comes down to..
2. Integral Membrane Proteins
Bound ribosomes also synthesize proteins that become embedded in the ER membrane. Many of these proteins remain in cellular membranes, while others travel through the Golgi and other endomembrane compartments before reaching the plasma membrane or another destination Less friction, more output..
Membrane proteins are especially important because they function as:
- Receptors that detect hormones, neurotransmitters, and other signals.
- Ion channels that control the movement of ions across membranes.
- Transporters that move glucose, amino acids, and other substances.
- Adhesion molecules that help cells recognize and bind to one another.
- Cell-surface markers used for cell identification and immune recognition.
- Enzymes embedded in biological membranes.
A membrane protein may have several possible arrangements. That said, it may span the membrane once, cross it several times, or be attached to the cytosolic face as a peripheral protein. Still, the part responsible for transmembrane signaling or transport often contains hydrophobic regions that must be inserted into the lipid bilayer.
The orientation of a membrane protein is established during its synthesis and insertion into the ER. Once correctly inserted, the protein can be transported to the plasma membrane while preserving its orientation.
3. Lysosomal Proteins
Proteins that function inside lysosomes are synthesized by bound ribosomes on the rough ER. They are then transported through the Golgi apparatus, where they receive specific carbohydrate modifications. These modifications act as sorting signals that direct the proteins to lysosomes.
Examples include:
- Acid hydrolases that break down carbohydrates, proteins, fats, and nucleic acids.
- Lysosomal membrane proteins that help maintain the organelle’s acidic environment.
- Proteins involved in lysosome formation and the movement of material into lysosomes.
A useful modification is mannose-6-phosphate, which is added to many lysosomal enzymes in the Golgi. This modification helps direct them toward lysosomes. If the relevant sorting machinery is defective, lysosomal enzymes may be released or sent elsewhere, potentially contributing to lysosomal storage disorders.
4. Proteins of the Golgi Apparatus and Endosomal System
Bound ribosomes synthesize proteins that function in the Golgi apparatus, endosomes, and related vesicles. These include:
- Golgi-resident enzymes.
- Vesicle-trafficking proteins.
- Endosomal sorting proteins.
- Receptors involved in moving materials between cellular compartments.
- Enzymes involved in the processing of proteins and lipids.
The Golgi modifies and sorts proteins arriving from the ER. It also sends some proteins back to the ER, sends others to lysosomes, and packages others for secretion or delivery to the plasma membrane.
5. Resident Proteins of the Endoplasmic Reticulum
The ER lumen contains its own resident proteins, and these are also synthesized by bound ribosomes. ER-resident proteins commonly include:
- Chaperones that help newly synthesized
