The tri-snRNP specific protein FgSnu66 is functionally related to FgPrp4 kinase in Fusarium graminearum

Deletion of Prp4, the only kinase among spliceosome components, is not lethal in Fusarium graminearum but Fgprp4 mutants have severe growth defects and produced spontaneous suppressors. To identify novel suppressor mutations of Fgprp4, we sequenced the genome of suppressor S37 that was normal in growth but only partially recovered for intron splicing and identified a tandem duplication of 9-aa in the tri-snRNP component FgSNU66. Among the 19 additional suppressor strains found to have mutations in FgSNU66 (out of 260 screened), five had the same 9-aa duplication event with S37 and another five had the R477H/C mutation. The rest had nonsense or G-to-D mutations in the C-terminal 27-aa (CT27) region of FgSnu66, which is absent in its yeast ortholog. Truncation of this C-terminal region reduced the interaction of FgSnu66 with FgHub1 but increased its interaction with FgPrp8 and FgPrp6. Five phosphorylation sites were identified in FgSnu66 by phosphoproteomic analysis and the T418A-S420A-S422A mutation was shown to reduce virulence. Overall, our results showed that mutations in FgSNU66 can suppress deletion of Fgprp4, which has not been reported in other organisms, and the C-terminal tail of FgSnu66 plays a role in its interaction with key tri-snRNP components during spliceosome activation.