Geometry of the protein S4 from Escherichia coli ribosomes

The shape of protein S4 from Escherichia coli ribosomes in solution was determined by hydrodynamic methods and low-angle X-ray scattering. The molecular weight of 24000 determined by low-angle X-ray scattering is within 3% of that found by sedimentation equilibrium analysis and 8% of that determined by amino acid sequence work. The radius of gyration of 3.36 nm, the radius of gyration of the cross section of 0.41 nm and the hydrodynamic studies revealed that protein S4 is not spherical, but rather has a markedly extended shape. Calculations of different conformations, e.g. random coil, based on the parameters evaluated from hydrodynamic methods, revealed a rod-like structure of S4 with a length of 14 nm and a diameter of 1 nm. This is supported by a model of an equivalent scattering particle of uniform density based on all parameters obtained in this study.