Identification of proteins at the binding site for protein S1 in 70 S ribosomes and 30 S subunits by cross-linking with 2-iminothiolane.

Escherichia coli 30 S ribosomal subunits and 70 S ribosomes were treated with 2-iminothiolane and oxidized to promote the formation of intermolecular disulfide bonds between neighboring proteins. After alkylation of the unreacted sulfhydryl groups, the mixture of ribosomal proteins was extracted with acetic acid and analyzed by 2-dimensional polyacrylamide gel electrophoresis using nonequilibrium pH gradient gel electrophoresis in the first dimension and a sodium dodecyl sulfate gel in the second. The analysis revealed only 3 protein dimers that contained ribosomal protein S1. The identification of the protein partner of protein S1 in each dimer was accomplished by radioiodination of the proteins extracted from the gel, 2-dimensional polyacrylamide/urea gel electrophoresis, and radioautography. The results indicate that proteins S2, S10, and S18 are cross-linked to protein S1. No other cross-links involving S1 were detected.