24 Mammalian Ribosomes: The Structure and the Evolution of the Proteins

Ribosomes are universal, essential, and complicated; they mediate protein synthesis in all organisms in our biosphere and thereby link genotype to phenotype. The imperative in research on ribosomes is to determine their structure and thus be able to account for their function. This led to the isolation, the characterization, and the determination of the structures of mammalian (rat) ribosomal proteins and nucleic acids (Wool 1979Wool 1986; Wool et al. 1990). Mammalian ribosomes are composed of two subunits that are designated by their sedimentation coefficients: The smaller is 40S and the larger is 60S. The subunits associate—they are held together by noncovalent bonds, perhaps by magnesium salt bridges—and form the functional 80S ribosome. The 40S subunit has a single molecule of RNA, designated 18S rRNA, and 33 proteins, whereas the 60S subunit has three molecules of RNA, designated 5S, 5.8S, and 28S rRNA, and 47 proteins. The covalent structures of the four species of rat rRNAs (5S, 5.8S, 18S, and 28S) have been established (Nazar et al. 1975; Aoyama et al. 1982; Chan et al. 1983, 1984; Torczyn-ski et al. 1983; Hadjiolov et al. 1984), and there are rational proposals for their secondary structures (Chan et al. 1984; Hadjiolov et al. 1984; Wool 1986; Raue et al. 1988). Eighty-two proteins have been isolated from rat ribosomes, and the complete amino acid sequences in 75 of these proteins have been either determined directly from the protein or deduced from the sequences of nucleotides in cDNAs (for references, see Table A1...