Large ribosomal subunit protein uL16 superfamily (Ribosomal_uL16_sf)
Type: Homologous_superfamily

This entry represents the large ribosomal subunit protein uL16 superfamily, which is widespread among all cellular organisms. Members of this entry contain a single structural domain with an α/β-hammerhead fold (

).

uL16 is an essential protein in the large ribosomal subunit of bacteria, mitochondria, and chloroplasts. Large subunits that lack uL16 are defective in peptidyl transferase activity, peptidyl-tRNA hydrolysis activity, association with the 30S subunit, binding of aminoacyl-tRNA and interaction with antibiotics. uL16 is required for the function of elongation factor P (EF-P), a protein involved in peptide bond synthesis through the stimulation of peptidyl transferase activity by the ribosome. Mutations in uL16 and the adjoining bases of 23S rRNA confer antibiotic resistance in bacteria, suggesting a role for uL16 in the formation of the antibiotic binding site. The GTPase RbgA (YlqF) is essential for the assembly of the large subunit, and it is believed to regulate the incorporation of uL16. Bacterial, archaeal, and eukaryotic uL16 proteins exhibit structural differences at the N terminus .

Ribosomes are the particles that catalyse mRNA-directed protein synthesis in all organisms. The codons of the mRNA are exposed on the ribosome to allow tRNA binding. This leads to the incorporation of amino acids into the growing polypeptide chain in accordance with the genetic information. Incoming amino acid monomers enter the ribosomal A site in the form of aminoacyl-tRNAs complexed with elongation factor Tu (EF-Tu) and GTP. The growing polypeptide chain, situated in the P site as peptidyl-tRNA, is then transferred to aminoacyl-tRNA and the new peptidyl-tRNA, extended by one residue, is translocated to the P site with the aid the elongation factor G (EF-G) and GTP as the deacylated tRNA is released from the ribosome through one or more exit sites . About 2/3 of the mass of the ribosome consists of RNA and 1/3 of protein. The proteins are named in accordance with the subunit of the ribosome which they belong to the small (S1 to S31) and the large (L1 to L44). Usually they decorate the rRNA cores of the subunits.

Many ribosomal proteins, particularly those of the large subunit, are composed of a globular, surfaced-exposed domain with long finger-like projections that extend into the rRNA core to stabilise its structure. Most of the proteins interact with multiple RNA elements, often from different domains. In the large subunit, about 1/3 of the 23S rRNA nucleotides are at least in van der Waal's contact with protein, and L22 interacts with all six domains of the 23S rRNA. Proteins S4 and S7, which initiate assembly of the 16S rRNA, are located at junctions of five and four RNA helices, respectively. In this way proteins serve to organise and stabilise the rRNA tertiary structure. While the crucial activities of decoding and peptide transfer are RNA based, proteins play an active role in functions that may have evolved to streamline the process of protein synthesis. In addition to their function in the ribosome, many ribosomal proteins have some function 'outside' the ribosome .