Interactions between Giardia duodenalis Sm proteins and their association with spliceosomal snRNAs

Giardia duodenalis is a parasite that colonises the intestines of humans and other vertebrates, causing diarrhoea and poor nutrient absorption. G. duodenalis is sometimes considered an early diverging eukaryote, and its genome exhibits simplified molecular machinery for many cellular processes, which makes it an interesting model to study. The spliceosome, one of the most complex molecular machines in the eukaryotic cell, is responsible for intron excision and exon splicing. Just over a decade ago, it was believed that the G. duodenalis genome did not contain introns or undergo splicing. Research now shows that this speculation was incorrect and that uncommon mechanisms, such as trans-splicing from different genes, occur. In silico studies of the parasite suggest the possibility of a simplified spliceosome and spliceosomal small nuclear RNA (snRNA) candidates; however, none of these components have been identified in vivo. Here, we developed a strategy to study the in vivo expression, interactions and localisation of these spliceosome components in G. duodenalis. Haemagglutinin (HA)-tagged SmB and SmD3 proteins, which form part of the spliceosome core, were overexpressed in the parasite. Immunoprecipitation with anti-HA revealed that the SmD3 protein is associated with the proteins SmB, SmD1, SmD2, SmE and SmF in vivo. In addition, the U1, U2 and U4 snRNA candidates reported previously were found in the protein complex, suggesting that these molecules are spliceosomal snRNAs of G. duodenalis and they contained a 2,2,7-trimethylguanosine modification at their 5′ end. Our data indicate that the actively expressed spliceosome in G. duodenalis is similar to that of highly evolved protists and higher animals.