Binds the phosphorylated C-terminal domain (CTD) of the largest subunit of RNA polymerase II and functions as a scaffold for RNA processing machineries (Probable). May be involved in pre-mRNA splicing (Probable). Interacts (via the WW domains) with the phosphorylated C-terminal domain of NRPB1 (via CTD domain). Highly expressed in roots (PubMed:19467629). Expressed in shoots, rosette leaves, cauline leaves, stems and flowers (PubMed:19467629). No visible phenotype, probably due to the redundancy with PRP40B and PRP40C. Baeckstroem et al identified PRP40A in a Mediator complex pull-down assay and suggested that PRP40A could be a plant specific component of the Mediator complex (PubMed:17560376). However, no experimental evidence has been brought so far to confirm this hypothesis (Probable). Belongs to the PRPF40 family.