80S

80S refers to the eukaryotic cytosolic ribosome, a large ribonucleoprotein complex that catalyzes protein synthesis. The name derives from its sedimentation coefficient in ultracentrifugation, approximately 80 Svedberg units, which reflects a combination of size, mass, and shape rather than a simple mass measure.

It consists of two unequal subunits, 60S and 40S. The 60S subunit contains the 28S, 5.8S, and 5S rRNAs and about 49 proteins; the 40S subunit carries the 18S rRNA and about 33 proteins. The subunits assemble on messenger RNA during translation and form the tRNA binding sites A, P, and E.

The 80S ribosome translates mRNA into a polypeptide. The small 40S subunit decodes codons and recruits transfer RNA, while the large 60S subunit catalyzes peptide bond formation via peptidyl transferase activity in its ribosomal RNA. Initiation, elongation, and termination involve multiple eukaryotic initiation, elongation, and termination factors that coordinate the process.

In most eukaryotic cells, 80S ribosomes reside in the cytoplasm. Organelles such as mitochondria and chloroplasts have their own ribosomes, which often differ in size (commonly around 55S or 70S), reflecting their distinct evolutionary origins and translational machinery.

Evolutionarily, 80S ribosomes arose from ancestral bacterial ribosomes through endosymbiosis. They share core features with prokaryotic 70S ribosomes but exhibit eukaryote-specific expansions and a larger complement of ribosomal proteins.

Clinical and research relevance centers on protein synthesis and gene expression. Because they synthesize essential cellular proteins, 80S ribosomes are studied to understand disease mechanisms, and inhibitors that affect eukaryotic translation (such as cycloheximide) illustrate the distinction from antibiotics that target 70S ribosomes.