The human large subunit ribosomal protein L36A-like contacts the CCA end of P-site bound tRNA

Periodate-oxidized tRNA (tRNAox), the 2′,3′-dialdehyde derivative of tRNA, was used as a zero-length active site-directed affinity labeling reagent, to covalently label proteins at the binding site for the 3′-end of tRNA on human 80S ribosomes. When human 80S ribosomes were reacted with tRNAAspox positioned at the P-site, in the presence of an appropriate 12 mer mRNA, a set of two tRNAox-labeled ribosomal proteins (rPs) was observed. The majorily labeled protein was identified as the large subunit rP L36a-like (RPL36AL) by means of mass spectrometry. Intact tRNAAsp competed with tRNAAspox for the binding to the P-site, by preventing tRNA-protein cross-linking with RPL36AL. Altogether, the data presented in this report are consistent with the presence of RPL36AL at or near the binding site for the CCA end of the tRNA substrate positioned at the P-site of human 80S ribosomes. It is the first time that a ribosomal protein is found in an intimate contact (i.e. at a zero-distance) with a nucleotide of the conserved CCA terminus of P-site tRNA which is the substrate of peptidyl transferase reaction. RPL36AL which is strongly conserved in eukaryotes belongs to the L44e family of rPs, a representative of which is Haloarcula marismortui RPL44e.