Cleaves the 2'-5' phosphodiester linkage at the branch point of lariat intron pre-mRNAs after splicing and converts them into linear molecules that are subsequently degraded. It thereby facilitates ribonucleotide turnover. It also participates in Ty1 retrovirus-like transposition via an RNA lariat intermediate in cDNA synthesis. Present with 1130 molecules/cell in log phase SD medium. Belongs to the lariat debranching enzyme family.