PROTEIN PHOSPHATASE 2A B'η drives spliceosome subunit dephosphorylation to mediate alternative splicing following heat stress.

Dephosphorylation of spliceosome components is an essential regulatory step for intron removal from pre-mRNA, thereby controlling gene expression. However, the specific phosphatase responsible for this dephosphorylation step has not been identified. Here, we show that Arabidopsis thaliana (Arabidopsis) PROTEIN PHOSPHATASE 2A B'η (PP2A B'η), a B subunit of PP2A, interacts with the splicing factors PRP18a, PRP16, and RH2 and facilitates their dephosphorylation by recognizing substrates through a conserved binding motif. This dephosphorylation is crucial for proper splicing of retained introns in heat stress-responsive genes, which is mediated by the PP2A interactor PRE-MRNA PROCESSING FACTOR 18a. Genetic inactivation of PP2A B'η abolished thermotolerance during seed germination and resulted in widespread intron retention in heat stress-responsive genes. Conversely, overexpression of PP2A B'η conferred enhanced thermotolerance, accompanied by the efficient removal of retained introns under heat stress. We demonstrate that a B regulatory subunit of PP2A plays a central role in dephosphorylating spliceosome components, regulating alternative splicing, facilitating acclimation to heat stress, and targeting specific spliceosome subunits that activate pre-mRNA splicing.