PROTEINS ON RIBOSOME SURFACE - MEASUREMENTS OF PROTEIN EXPOSURE BY HOT TRITIUM BOMBARDMENT TECHNIQUE

The hot tritium bombardment technique [Goldanskii, V. I., Kashirin, I, A. Shishkov, A. V., Baratova, L. A. & Grebenshchikov, N. I. (1988) J. Mel. Biol. 201, 567-574] has been applied to measure the exposure of proteins on the ribosomal surface, The technique is based on replaceme nt of hydrogen by high energy tritium atoms in thin surface layer of m acromolecules. Quantitation of tritium radioactivity of each protein h as revealed that proteins S1, S4, S5, S7, S18, S20, and S21 of the sma ll subunit, and proteins L7/L12, L9, L10, L11, L16, L17, L24, and L27 of the large subunit are well exposed on the surface of the Escherichi a coli 70 S ribosome, Proteins S8, S10, S12, S16, S17, L14, L20, L29, L30, L31, L32, L33, and L34 have virtually no groups exposed on the ri bosomal surface, The remaining proteins are found to be exposed to les ser degree than the well exposed ones, No additional ribosomal protein s was exposed upon dissociation of ribosomes into subunits, thus indic ating the absence of proteins on intersubunit contacting surfaces.