InterPro domain: IPR038579

A number of eukaryotic ribosomal proteins can be grouped on the basis ofsequence similarities. These proteins have 82 to 87 amino acids. The amino termini are all N alpha-acetylated. The N-terminal halves of the protein molecules are highly conserved in contrast to the carboxy-terminal parts [ 1 ].

This entry represents the ribosomal protein S21 (RpS21). It bounds to native 40S ribosomal subunits in a salt-labile association and is absent from polysomes, indicating that it acts as a translation initiation factor rather than as a core ribosomal protein [ 2 ].

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 [ 3 , 4 ]. 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.

1. Primary structures of ribosomal protein YS25 from Saccharomyces cerevisiae and its counterparts from Schizosaccharomyces pombe and rat liver. Biochemistry 24, 7418-23
2. Down-regulation of RpS21, a putative translation initiation factor interacting with P40, produces viable minute imagos and larval lethality with overgrown hematopoietic organs and imaginal discs. Mol. Cell. Biol. 19, 2308-21
3. Atomic structures at last: the ribosome in 2000. Curr. Opin. Struct. Biol. 11, 144-54
4. The ribosome in focus. Cell 104, 813-6