Purification of the spliced leader ribonucleoprotein particle from Leptomonas collosoma revealed the existence of an Sm protein in trypanosomes - Cloning the SmE homologue

Trans-splicing in trypanosomes involves the addition of a common spliced le ader (SL) sequence, which is derived from a small RNA, the SL RNA, to all m RNA precursors. The SL RNA is present in the cell in the form of a ribonucl eoprotein, the SL RNP, Using conventional chromatography and affinity selec tion with 2'-O-methylated RNA oligonucleotides at high ionic strength, five proteins of 70, 16, 13, 12, and 8 kDa were co-selected with the SL RNA fro m Leptomonas collosoma, representing the SL RNP core particle. Under condit ions of lower ionic strength, additional proteins of 28 and 20 kDa were rev ealed. On the basis of peptide sequences, the gene coding for a protein wit h a predicted molecular weight of 11.9 kDa was cloned and identified as hom ologue of the cis-spliceosomal SmE. The protein carries the Sm motifs 1 and 2 characteristic of Sm antigens that bind to all known cis-spliceosomal ur idylic acid-rich small nuclear RNAs (U snRNAs), suggesting the existence of Sm proteins in trypanosomes, This finding is of special interest because t rypanosome snRNPs are the only snRNPs examined to date that are not recogni zed by anti-Sm antibodies. Because of the early divergence of trypanosomes from the eukaryotic lineage, the trypanosome SmE protein represents one of the primordial Sm proteins in nature.