The 5‐S RNA Binding Protein from Yeast (Saccharomyces cerevisiae) Ribosomes

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Abstract
The ribonucleoprotein complex between 5-S RNA and its binding protein (5-S RNA.cntdot.protein complex) of yeast ribosomes was released from 60-S subunits with 25 mM EDTA and the protein component was purified by chromatography on DEAE-cellulose. This protein, designated YL3 (MW = 36,000 on dodecylsulfate gels), was relatively insoluble in neutral solutions (pH 4-9) and migrated as 1 of 4 acidic 60-S subunit proteins when analyzed by the Kaltschmidt and Wittman 2-dimensional gel system. Amino acid analyses indicated lower amounts of lysine and arginine than most ribosomal proteins. Sequence homology was observed in the N terminus of YL3, and 2 prokaryotic 5-S RNA binding proteins, EL18 from Escherichia coli and HL13 from Halobacterium cutirubrum [H. salinarium]: Ala1-Phe2-Gln3-Lys4-Asp5-Ala6-Lys7-Ser8-Ser9-Ala10-Tyr11-Ser12-Ser13-Arg14-Phe15-Gln16-Tyr17-Pro18-Phe19-Arg20-Arg21-Arg22-Arg23-Glu24-Gly25-Lys26-Thr27-Asp28-Tyr29-Tyr35; of particular interest was homology in the cluster of basic residues (18-23). Since the protein contained 1 methionine residue it could be split into 2 fragments, CN1 (MW = 24,700) and CN2 (MW = 11,300) by CNBr treatment; the larger fragment originated from the N terminus. The N-terminal amino acid sequence of CN2 shared a limited sequence homology with an internal portion of a 2nd 5-S RNA binding protein from E. coli, EL5, and, based also on the MW of the proteins and studies on the protein binding sites in 5-S RNA, a model for the evolution of the eukaryotic 5-S RNA binding protein is suggested in which a fusion of the prokaryotic sequences may have occurred. Unlike the native 5-S RNA.cntdot.protein complex, a variety of RNA interated with the smaller CN2 fragment to form homogeneous ribonucleoprotein complexes; the CN1 fragment may confer specificity on the natural 5-S RNA-protein interaction.