The function of the N-terminal region of ribosomal protein S4

Abstract

Single ribosomal protein-16 S RNA complexes were examined for their sensitivity to trypsin digestion under conditions which maintain specific protein-RNA interactions. Proteins S7 and S13 are completely digested and removed from the complex while proteins S8, S15, S17 and S20 are protected from trypsin attack by the 16 S RNA. Protein S4 is partially protected yielding a fragment of a molecular weight of about 17,700. These results are in agreement with similar experiments done with different digestion conditions by Schulte et al. (1975). We have determined that the portion of protein S4 removed by the trypsin is the N-terminus up to residues 43 to 46. This conclusion is based on the following experimental results: (1) the tryptophan residue at position 167 remains a part of the S4 fragment after trypsin digestion; (2) the cysteine residue at position 31 is removed by trypsin digestion; (3) tryptide fingerprints indicate that the tyrosine at position 50 is not lost by trypsin digestion; (4) the loss of 4800 daltons is equivalent to approximately 44 amino acid residues. The role the N-terminus of protein S4 plays in ribosome assembly and function was investigated. Protein S4 fragment was mixed with 20 purified 30 S ribosomal proteins excluding protein S4 and reconstituted with 16 S RNA. An incomplete particle was produced. The proteins completely missing are S2, S10, S18 or S19 and S21. We postulate that these proteins interact either directly or indirectly with the N-terminal region of S4. Correlating this hypothesis with the electron microscopic results of Tischendorf et al. (1975), we also suggest that the N-terminus of protein S4 resides in the “head” region of the electron microscope model.