Compact globular structure of Thermus thermophilus ribosomal protein S1 in solution -: Sedimentation and calorimetric study

Ribosomal protein S1 of Thermus thermophilus overexpressed in Escherichia coli cells has been isolated and subjected to studies by analytical sedimentation and differential scanning microcalorimetry techniques. It has been demonstrated that the protein of 60 kDa sediments at s(20,w)(0) = 4.6 S and has the diffusion coefficient D-20,w(0) = 6.7 x 10(-7) cm(2)/s in 25 mM HEPES-NaOH buffer, pH 7.5 (similarly to bovine serum albumin of 66 kDa that sediments at s(20,w)(0) = 4.4 S and D-20,w(0) = 6.0 x 10(-7) cm(2)/s), indicating its compact globular conformation under these conditions. The microcalorimetry study has shown the presence of a cooperative tertiary structure melting at 90 degreesC, but with several (probably three) independent cooperative domains. In the presence of 100 mM NaCl the protein becomes more asymmetric (s(20,w)(0) = 3.1 S) but does not lose its cooperativity and thermostability, this suggesting just the weakening of interdomain ionic interactions. The compact globular conformation of protein S1 seems to be most likely within the ribosome.