Protein synthesis requires accurate translation of the nucleotide sequence of messenger RNA (mRNA) to the amino acid sequence of a protein. This translation of mRNA to protein is carried out by the ribosome and transfer RNA (tRNA), along with other protein factors. In past years, studies on the structure of the ribosome have led us to understand this complex process of protein synthesis. The ribosome consists of two subunits, each of which is made up of ribosomal RNA (rRNA) and many ribosomal proteins. Structurally, ribosomes of prokaryotes and eukaryotes vary by the types of rRNA and protein molecules found in them. The prokaryotic 70S ribosome has a small 30S and a large 50S subunit. The 30S subunit consists of one 16S molecule of rRNA and about 21 proteins, while the 50S subunit consists of two rRNAs (5S and 23S) and 31 proteins. The eukaryotic 80S ribosome has a small 40S and a large 60S subunit. The 40S subunit consists of one 18S molecule of rRNA and about 33 proteins, whereas the 60S consists of three rRNAs (5S, 28S and 5.8S) and about 50 proteins [1
During protein synthesis, the small ribosomal subunit plays a role in accurate codon-anticodon recognition between the mRNA and tRNA molecules, while the large subunit is mainly involved in the peptide bond formation of the growing amino acid chain. In addition, structural studies of the ribosome have now revealed that they are also involved in functions such as the translocation of tRNA and mRNA on the ribosome [2
Apart from protein synthesis, many of the ribosomal proteins are shown to be involved in other cellular functions, independent of the ribosome [3
]. Their first extra-ribosomal activity was observed for S1, as a replicase in the RNA phages, and numerous extra-ribosomal functions of these proteins have subsequently been discovered. This bifunctional tendency of ribosomal proteins can be explained by theories postulating the pre-existence of the ribosomal proteins as independent molecules before forming the components of the ribosome [3
]. Another interesting functional aspect of the ribosomal proteins is their regulation. These proteins are shown to affect the mechanisms of development, apoptosis and ageing during their altered expression levels. In this review, information on the extra-ribosomal roles of these proteins is provided, along with information about their specific regulation in different cellular functions. Detailed lists of all functions and regulation are presented as Tables S1 (Table ) and S2 (Table ).
Function and regulation of eukaryotic small subunit ribosomal proteins
Function and regulation of eukaryotic large subunit ribosomal proteins