Small ribosomal subunit protein uS12, eukaryotic/archaeal (Ribosomal_uS12_euk/arc)
Type: Family

Ribosomal protein uS12 is one of the proteins from the small ribosomal subunit. In Escherichia coli, uS12 is known to be involved in the translation initiation step. It is a very basic protein of 120 to 150 amino-acid residues. uS12 belongs to a family of ribosomal proteins which are grouped on the basis of sequence similarities. This family represents the eukaryotic and archaeal homologues of bacterial ribosomal protein uS12. This protein was known as S23 in eukaryotes and as either S12 or S23 in the Archaea .

In eukaryotes, uS12 is located at the interface of the large and small ribosomal subunits, adjacent to the decoding centre. It interacts with domain III of the eukaryotic elongation factor 2 (eEF2), which catalyses the translocation of the growing peptidyl-tRNA to the P site to make room for the next aminoacyl-tRNA at the A (acceptor) site. Through its interaction with eEF2, uS12 may play an important role in translocation . uS12 functions as control element for the rRNA- and tRNA-driven movements of translocation. uS12 has been also implicated in translation accuracy. Antibiotics such as streptomycin bind uS12 and cause the ribosome to misread the genetic code.

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 .