Living organisms depend on the continuous production of proteins, and the cellular machinery responsible for this is the ribosome. These structures act as molecular factories, translating genetic instructions into functional proteins. This article will focus on the 80S ribosome, a type found in a major group of organisms.
Defining 80S Ribosomes
The “80S” designation comes from the ribosome’s sedimentation coefficient, measured in Svedberg units (S), which reflects the rate a particle settles during ultracentrifugation. These ribosomes are the protein synthesizers of eukaryotic cells, including those of animals, plants, fungi, and protists. Within these cells, 80S ribosomes have two main locations. They can be found floating freely in the cytoplasm to synthesize proteins that function within the cytosol, or bound to the endoplasmic reticulum to produce proteins for secretion, membrane insertion, or delivery to organelles.
Structural Composition of 80S Ribosomes
A functional 80S ribosome is composed of two unequal subunits that perform translation: the large 60S subunit and the small 40S subunit. Svedberg units are not additive because the sedimentation rate is affected by shape and surface area, which is why 60S and 40S combine to form an 80S particle.
Each subunit is an assembly of ribosomal RNA (rRNA) and ribosomal proteins. The small 40S subunit contains a single 18S rRNA molecule and about 33 proteins. The larger 60S subunit contains three rRNA molecules (28S, 5.8S, and 5S) and around 49 proteins.
When the two subunits join, the small subunit provides a cleft for binding messenger RNA (mRNA). The assembled ribosome features three sites where transfer RNA (tRNA) molecules operate: the A (aminoacyl) site binds an incoming tRNA, the P (peptidyl) site holds the growing polypeptide chain, and the E (exit) site discharges used tRNA.
The Process of Protein Synthesis by 80S Ribosomes
Protein synthesis, or translation, is a process managed by the 80S ribosome in three phases. The first is initiation, where the small 40S subunit binds to an mRNA molecule and scans for a start codon. Upon finding this signal, the large 60S subunit joins to form a complete 80S ribosome.
The second phase is elongation, a cyclical process where the protein chain is built. The ribosome moves along the mRNA one codon at a time. For each codon, a transfer RNA (tRNA) with a complementary anticodon arrives at the A site, carrying the corresponding amino acid. The ribosome then catalyzes a peptide bond, linking the new amino acid to the growing polypeptide chain at the P site. The structure then shifts, moving the empty tRNA to the E site for release.
This elongation cycle repeats until the ribosome encounters a stop codon on the mRNA, signaling termination. Release factor proteins recognize the stop codon and bind to the A site. This causes the ribosome to sever the bond holding the completed polypeptide chain, which is then released. The ribosomal subunits and mRNA dissociate, ready to be reused.
Key Distinctions from 70S Ribosomes
A primary distinction is their distribution: 80S ribosomes are in the cytoplasm of eukaryotic cells, while 70S ribosomes operate in prokaryotes like bacteria. However, 70S ribosomes are also found inside eukaryotic mitochondria and chloroplasts, reflecting the endosymbiotic origin of these organelles.
Structural differences begin with size and subunit composition. The 80S ribosome is larger, with 60S and 40S subunits, while the 70S ribosome is smaller, with 50S and 30S subunits. These size differences result from variations in their rRNA and protein components, such as the 18S rRNA in the 80S small subunit versus the 16S rRNA in the 70S small subunit.
These structural distinctions have medical implications for antibiotic sensitivity. Many antibiotics selectively inhibit bacterial 70S ribosomes without affecting the host’s 80S ribosomes. Drugs like tetracycline and erythromycin target the 70S ribosome, halting bacterial protein synthesis without harming the patient’s cells. Conversely, substances like cycloheximide inhibit the 80S ribosome and are used in labs to study protein synthesis in eukaryotes.