Brr2 is a splicing fidelity factor

Many spliceosomal DExD/H box helicases act as fidelity factors during pre-mRNA splicing, promoting on-pathway interactions while simultaneously minimizing errors. Mutations linked to Retinitis Pigmentosa (RP), a form of heritable blindness, map to key domains of spliceosomal helicase Brr2 (SNRNP200 in humans). Previous data show that such mutations negatively impact spliceosome activation, likely due to defects in brr2-RP RNA binding, helicase, and ATPase activities. Furthermore, data from human reporter constructs suggest that brr2-RP might impact 5' splice site selection. Here we undertake a systematic analysis of brr2-RP effects on splicing fidelity. We show that a subset of brr2-RP mutants exhibit intron retention in vivo. Furthermore, brr2-RP mutants display hyperaccurate and/or error-prone splicing of a variety of splicing reporters. Branch-site fidelity is particularly impacted in this reporter assay. In addition, multiple brr2-RP alleles genetically interact with prp16 alleles known to impact the fidelity of branch site selection. Together these data implicate Brr2 in the fidelity of branch-site selection, and suggest that RP results not just from defects in spliceosome activation, but also from fidelity defects arising throughout the splicing cycle and in splicing fidelity.