The rice blast fungus SR protein 1 regulates alternative splicing with unique mechanisms.

Serine/arginine-rich (SR) proteins are well known as splicing factors in humans, model animals and plants. However, they are largely unknown in regulating pre-mRNA splicing of filamentous fungi. Here we report that the SR protein MoSrp1 enhances and suppresses alternative splicing in a model fungal plant pathogen Magnaporthe oryzae. Deletion of MoSRP1 caused multiple defects, including reduced virulence and thousands of aberrant alternative splicing events in mycelia, most of which were suppressed or enhanced intron splicing. A GUAG consensus bound by MoSrp1 was identified in more than 94% of the intron or/and proximate exons having the aberrant splicing. The dual functions of regulating alternative splicing of MoSrp1 were exemplified in enhancing and suppressing the consensus-mediated efficient splicing of the introns in MoATF1 and MoMTP1, respectively, which both were important for mycelial growth, conidiation, and virulence. Interestingly, MoSrp1 had a conserved sumoylation site that was essential to nuclear localization and enhancing GUAG binding. Further, we showed that MoSrp1 interacted with a splicing factor and two components of the exon-joining complex via its N-terminal RNA recognition domain, which was required to regulate mycelial growth, development and virulence. In contrast, the C-terminus was important only for virulence and stress responses but not for mycelial growth and development. In addition, only orthologues from Pezizomycotina species could completely rescue defects of the deletion mutants. This study reveals that the fungal conserved SR protein Srp1 regulates alternative splicing in a unique manner. Author summary: The primary roles of serine/arginine-rich (SR) proteins in pre-mRNA splicing have been well established in humans, model animals and plants. Some studies have suggested that fungal SR proteins are also involved in pre-mRNA splicing, but direct evidence is lacking to illustrate how they regulate pre-mRNA splicing in fungi. Here we describe that a fungal conserved SR protein MoSrp1 in the model plant fungal pathogen Magnaporthe oryzae regulates alternative pre-mRNA splicing with unique mechanisms. MoSrp1 enhances and suppresses alternative splicing by binding to the GUAG consensus in target pre-mRNAs. The GUAG consensus per se enhances and suppresses the intron splicing efficiency of MoATF1 and MoMTP1, both of which are important for mycelial growth, conidiation, and virulence. We also show that MoSrp1 is SUMOylatied for its nuclear localization and enhancing the GUAG binding capacity. Furthermore, we demonstrate that the N-terminal RNA recognition domain of MoSrp1 interacts with a splicing factor and two components of the exon-joining complex and is required for mycelial growth, development and virulence, and that the C-terminus was important only for virulence and stress responses but not for mycelial growth and development. [ABSTRACT FROM AUTHOR]