Ribosome the Protein Factory: Function and Structure

The ribosome is responsible for protein synthesis in cells and is often referred to as the protein factory. Ribosomes can be found in both prokaryotic and eukaryotic cells, and their main function is to convert the genetic information encoded in messenger RNA (mRNA) into functional proteins.

Ribosomes are not surrounded by a membrane. They consist of two subunits, commonly referred to as the large subunit and the small subunit, which are suspended in the cytoplasm. These subunits are composed of ribosomal RNA (rRNA) and proteins. Only when they are involved in protein synthesis, ribosomes appear in their complete form. The ribosomes in eukaryotes are larger and have a different structure compared to those in prokaryotes.

Bound ribosome and free ribosome

There are multiple forms of ribosomes within the eukaryotes. Some ribosomes are attached to the endoplasmic reticulum or the nuclear envelope and are known as bound ribosomes. Others are suspended in the cytoplasm and are called free ribosomes. These two types of ribosomes are structurally identical. Ribosomes are also present in mitochondria and chloroplasts, and their structure is more similar to ribosomes found in prokaryotes. They are responsible for synthesizing a part of proteins in mitochondria or chloroplasts.

Ribosomes serve as the site for protein synthesis within the cell. Proteins synthesized by bound ribosomes enter the endoplasmic reticulum where they undergo folding and modifications. They are then transported to the Golgi apparatus for further processing. These proteins are eventually secreted from the cell, such as antibodies and protein hormones. Bound ribosomes also play a role in making integral proteins and some proteins that enter organelles, such as hydrolytic enzymes in lysosomes or vacuoles. Proteins synthesized by free ribosomes primarily function within the cytoplasm. These proteins have various functions, including enzymes and structural proteins, such as those that make up the cytoskeleton. Therefore, ribosomes are abundant in rapidly dividing and secreting cells.

Whether they are bound or free, a group of ribosomes bound to a mRNA chain, which is referred as a polyribosome or polysome. These ribosomes move along the mRNA to produce proteins simultaneously, which act like machines on an assembly line (mRNA) and rise the productivity.

The brief process of protein synthesis by ribosomes

Prior to protein synthesis, the large and small subunits and initiator tRNA assemble near the start codon of the mRNA to form a complete ribosome in the cytoplasm.

The cytoplasm contains multiple types of tRNA. One end of tRNA is attached to a specific amino acid, while the other end recognizes the codons on the mRNA. tRNAs continuously carry amino acids into the ribosome for the synthesis of polypeptides under the catalytic action of enzymes, while the ribosome moves continuously from 5' to 3' direction. When the ribosome encounters a stop codon, synthesis stops, and the peptide chain is released. After completing translation, the large and small subunits of the ribosome separate once again.

Frequently Asked Questions

How do ribosomes enter the endoplasmic reticulum (ER)?

Bound ribosomes and free ribosomes are structurally identical. Whether they are suspended in the cytoplasm or associated with the endoplasmic reticulum (ER) depends on whether the mRNA being transcripted has an ER-targeting signal sequence, which is located after the start codon. This sequence is translated by the ribosome to generate a signal peptide. Ribosomes that can synthesize this signal peptide will enter the endoplasmic reticulum, otherwise they will be scattered in the cytoplasm.

The signal peptide bind to a signal recognition particle (SRP) in the cytoplasm. The SRP occupies the A site of the ribosome to halt translation temporarily. The SRP-ribosome complex is recognized by SRP receptors on the rough endoplasmic reticulum and attaches to the ER. Subsequently, the SRP dissociates from the receptor, while the ribosome stays in the ER to synthesize polypeptide that will undergo further processing within the ER. Once the signal peptide has fulfilled its role, it is removed by enzymes in the ER. When synthesis is complete, the polypeptide chain dissociates from the ribosome, and the ribosome disassembles into its subunits, leaving the ER and reentering the cytoplasm.