Protein S1 from Escherichia coli Ribosomes: An Improved Isolation Procedure and Shape Determination by Small‐Angle X‐Ray Scattering

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Abstract
Ribosomal protein S1 from E. coli was studied in solution by small-angle X-ray scattering and the following parameters were obtained: the radius of gyration R = 8.0 .+-. 0.2 nm; largest diameter D = 28 nm; MW = (8-9) .times. 104. The data also yielded (with the assumption of a rigid particle with almost constant electron density) 2 radii of gyration of cross-section Rq1 = 2.5 .+-. 0.1 nm and Rq2 = 1.05 .+-. 0.05 nm and molecular volume = 140 nm3. The experimental scattering curve of S1 was compared with the theoretical scattering curves for several rigid triaxial homogeneous bodies and the closest fit was given by that of a flat elliptical cylinder with the dimensions of 4.5 nm and 0.88 nm for the 2 semiaxes and 26.5 nm for height. The results from the present X-ray scattering studies and those from limited proteolytic digestion of protein S1 support the notion that the structure of protein S1 is organized into 2 distinct subdomains within its elongated overall shape. Protein S1 was purified for this study by an efficient procedure which yielded 12 mg S1/g ribosomes. The isolated protein was fully active in functional tests before and after X-ray irradiation.