Physical studies on the ribosomal protein S2 from the Escherichia coli 30S

The protein S2 has been isolated from the 30S subunit of Escherichia coli A19 ribosomes [Littlechild, J., & Malcolm, A.L. (1978) Biochemistry 17, 3363-3369]. This salt-extracted protein is soluble and does not aggregate at salt concentrations of 0.3-0.4 M as used under reconstitution conditions. This differs from the S2 protein extracted by the acetic acid and urea method. The molecular weight from sedimentation equilibrium was found to be 29 200, and the protein was found to have a S0(20,w) value of 2.36S. The apparent specific volume at 20 degrees C was 0.726 mL.g(-1), and the D0(20,2) was 7.37 x 10(-7) cm(2)s(-1). The value for intrinsic viscosity was found to be 6.42 mL.g(-1). An axial ratio of (5-6):1 for a prolate ellipsoid of revolution was estimated by using these parameters. The circular dichroism and proton magnetic resonance studies show that protein S2 has both substantial amounts of alpha helix and beta-pleated sheet in solution and appears as a "folded" protein and not a random coil structure.