Ribosomal Proteins

7.1. Diversity; Nomenclature

Each of the two ribosomal subunits contains many different proteins most of which are represented by only one copy per ribosome. This is a fundamental difference between the structurally asymmetric ribosomal ri-bonucleoprotein and symmetric viral nucleoproteins which are formed by the ordered packaging of many identical protein subunits. The discovery in the pioneering studies of Waller (1961, 1964) that the ribosome contains many nonidentical protein molecules established an important principle of the structural organization of ribosomes.

The best technique for analytically separating ribosomal proteins is gel electrophoresis. Even one-dimensional gel electrophoresis under denaturing conditions gives a considerable fractionation of ribosomal proteins by charge and molecular size. Moderately basic polypeptides predominate among ribosomal proteins from most organisms, although several neutral and acidic proteins are always present as well. The molecular masses of ribosomal proteins are usually in the range of 10,000 to 30,000 daltons. Just a few proteins have a greater size, up to about 50,000 or 60,000 daltons (these are two proteins of the large subunit of mammalian ribosomes and one protein of the small subunit of E. coli ribosomes, respectively). On the other hand, the large subunit of both prokaryotic and eukaryotic ribosomes contains several (3 to 6) low-molecular-mass proteins, or polypeptides, of only about 50 - 60 amino acid residues in length, or even less.The small (30S) subunit of prokaryotic ribosomes contains about 20 proteins, while there are around 30 in the large (50S) ribosomal subunit. Eukaryotic ribosomes contain a broader spectrum of proteins: the small (40S) subunit contains about 30 proteins, and the large (60S) about 50. Nearly all of these proteins are present as a single copy per ribosome.

Figure 7.1. Two-dimensional electrophoretic separation and nomenclature of proteins from Escherichia coli 30S ribosomal subunit (E. Kaltschmidt & H.G. Wittmann, Anal. Biochem. 36, 401-412, 1970; Proc. Natl. Acad. Sci. USA 67, 1276-1282, 1970). First direction (horizontal): 4% Polyacrylamide gel containing 8 M urea, pH 8.6; second direction (vertical, downward): 18 % polyacrylamide gel containing 6 M urea, pH 4.6.

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