Identification of the protein cross-linked to 3'-terminus of 5 S RNA in rat liver ribosomal 60 S subunits.

To cross-link the 3'-terminus of 5 S RNA to its neighbouring proteins, ribosomal 60 S subunits of rat liver were oxidized with sodium periodate and reduced with sodium borohydride. 5 S RNP was then isolated by EDTA treatment followed by sucrose density-gradient centrifugation and subjected to SDS-polyacrylamide gel electrophoresis. The protein with a slower mobility than the L5 protein, which was thought to be cross-linked 5 S RNP, was labeled with 125I, treated with RNAase, and analyzed by two-dimensional polyacrylamide gel electrophoresis, followed by radioautography. A radioactive spot located anodically from L5 protein was observed, suggesting that it is the L5 protein-oligonucleotide complex. When analyzed by SDS slab polyacrylamide gel electrophoresis followed by radioautography, the peptide pattern of the alpha-chymotrypsin digest of this 125I-labeled protein-oligonucleotide complex was similar to that of the digest of 125I-labeled L5 protein. The results indicate that L5 protein binds to the 3'-terminal region of 5 S RNA in rat liver 60 S subunits.