Human ribosomal protein eS1 is engaged in cellular events related to processing and functioning of U11 snRNA

Ribosomal proteins are involved in many cellular processes through interactions with various RNAs. Here, applying the photoactivatable-ribonucleoside-enhanced cross-linking and immunoprecipitation approach to HEK293 cells overproducing ribosomal protein (rp) eS1, we determined the products of RNU5A-1 and RNU11 genes encoding U5 and U11 snRNAs as the RNA partners of ribosome-unbound rp eS1. U11 pre-snRNA-associated rp eS1 was revealed in the cytoplasm and nucleus where rp eS1-bound U11/U12 di-snRNP was also found. Utilizing recombinant rp eS1 and 4-thiouridine-containing U11 snRNA transcript, we identified an N-terminal peptide contacting the U-rich sequence in the Sm site-containing RNA region. We also showed that the rp eS1 binding site on U11 snRNA is located in the cleft between stem-loops I and III and that its structure mimics the respective site on the 18S rRNA. It was found that cell depletion of rp eS1 leads to a decrease in the splicing efficiency of minor introns and to an increase in the level of U11 pre-snRNA with the unprocessed 3' terminus. Our findings demonstrate the engagement of human rp eS1 in events related to the U11 snRNA processing and to minor-class splicing. Contacts of rp eS1 with U5 snRNA in the minor pre-catalytic spliceosome are discussed.