Further studies on the identity of proteins at the subunit interface of the 70 S E. coli ribosome

The ability of 1 m NH₄Cl to detach iodinated 50 S ribosomal proteins from 50 S subunits and 70 S ribosomes was compared. High salt treatment was effective in preferentially releasing L16, L20, L24, L26, L27, L29, and L30 from the 50 S subunit. Similar but smaller effects were seen for L2, L6, L15, L19, L28, and L31. When these results are combined with several previous studies on accessibility, twelve 50 S proteins appear to be less exposed in the 70 S particle than in the free subunit, by more than one entirely different measure of accessibility. These twelve must be considered strong candidates for possible subunit interface proteins.Lactoperoxidase catalyzed iodination was used to probe the surface topography of active and reversibly inactivated 30 S subunits. The magnesium depleted inactive 30 S particle reproducibly incorporates more ¹²⁵I than the active subunit indicating that a conformational change, characterized by an opening or expansion of the 30 S particles, accompanies 30 S inactivation. Seven 30 S proteins, S5, S21, S4, S7, S10, S13, and S16 become more accessible to lactoperoxidase as a result of inactivation. These proteins are different from those known to become more accessible to lactoperoxidase as a result of the conformational reorganization accompanying subunit association, S3, S6, S9, and S18. Thus, although both inactive 30 S and 50 S-bound 30 S are more open or reactive compared with free active 30 S, the regions which are affected appear to be different.