Solution behavior of proteins L7/L12 from the 50S ribosomal subunit of Escherichia coli

The behavior of Escherichia coli 50S ribosomal subunit proteins L7/L12 has been investigated in ribosome reconstitution buffer, TMK buffer, by sedimentation equilibrium and analytical gel filtration. Contrary to previous reports that L7/L12 exists in solution solely as dimer species [Möller, W., Groene, A., Terhorst, C., & Amons, R. (1972) Eur J. Biochem. 25, 5], results presented here indicate that L7/L12 undergoes a monomer-dimer-tetramer self-association, with equal equilibrium constants of 3.5 x 10(4) M-1 obtained for the monomer-dimer and dimer-tetramer steps. These results yield standard Gibbs' free energies of -6.1 +/ 0.6 kcal/mol at 20 degrees C. The observed absence of temperature dependence of this interaction over the range 5-25 degrees C indicated a zero standard enthalpy of self-association. Gel filtration results are presented that confirm the highly elongated shape of the L7/L12 molecule. The data suggest the corresponding Stokes radii for the monomer, dimer, and tetramer are 21-23, 26-28, and 29-32 omicron A, respectively. The significance of these results is discussed.