Phosphorylation by Prp4 kinase releases the self-inhibition of FgPrp31 in Fusarium graminearum.

Prp31 is one of the key tri-snRNP components essential for pre-mRNA splicing although its exact molecular function is not well studied. In a previous study, suppressor mutations were identified in the PRP31 ortholog in two spontaneous suppressors of Fgprp4 mutant deleted of the only kinase of the spliceosome in Fusarium graminearum. To further characterize the function of FgPrp31 and its relationship with FgPrp4 kinase, in this study we identified additional suppressor mutations in FgPrp31 and determined the suppressive effects of selected mutations. In total, 28 of the 35 suppressors had missense or nonsense mutations in the C terminus 465-594 aa (CT130) region of FgPrp31. The other 7 had missense or deletion mutations in the 7-64 aa region. The nonsense mutation at R464 in FgPRP31 resulted in the truncation of CT130 that contains all the putative Prp4 kinase-phosphorylation sites reported in humans, and partially rescued intron splicing defects of Fgprp4. The CT130 of FgPrp31 displayed self-inhibitory interaction with the N-terminal 1-463 (N463) region, which was reduced or abolished by the L532P, D534G, or G529D mutation in yeast two-hybrid assays. The N463 region, but not full-length FgPrp31, interacted with the N-terminal region of FgBrr2, one main U5 snRNP protein. The L532P mutation in FgPrp31 increased its interaction with FgBrr2. In contrast, suppressor mutations in FgPrp31 reduced its interaction with FgPrp6, another key component of tri-snRNP. Furthermore, we showed that FgPrp31 was phosphorylated by FgPrp4 in vivo. Site-directed mutagenesis analysis showed that phosphorylation at multiple sites in FgPrp31 is necessary to suppress Fgprp4, and S520 and S521 are important FgPrp4-phosphorylation sites. Overall, these results indicated that phosphorylation by FgPrp4 at multiple sites may release the self-inhibitory binding of FgPrp31 and affect its interaction with other components of tri-snRNP during spliceosome activation.