Phosphorylation of Ribosomal Protein L18 Is Required for Its Folding and Binding

Abstract

Ribosomal protein L18 from Bacillus stearothermophilus (bL18) includes a previously unreported phosphoserine residue. The folded conformation of the protein is stabilized by the dianionic form of the phosphate group of that residue. In the absence of Mg , the pK a of the phosphate group is so high that the protein is not fully folded at pH 7. In the presence of Mg , its pK a drops significantly, and consequently the native conformation of bL18 becomes stable at pH 7 and the protein is able to bind to 5S rRNA. Dephosphorylated bL18 does not bind to 5S rRNA at neutral pH