Regulatory subunit of the poly(A)-nuclease (PAN) deadenylation complex, one of two cytoplasmic mRNA deadenylases involved in mRNA turnover. PAN specifically shortens poly(A) tails of RNA and the activity is stimulated by poly(A)-binding protein pab1. PAN deadenylation is followed by rapid degradation of the shortened mRNA tails by the CCR4-NOT complex. Deadenylated mRNAs are then degraded by two alternative mechanisms, namely exosome-mediated 3'-5' exonucleolytic degradation, or deadenlyation-dependent mRNA decaping and subsequent 5'-3' exonucleolytic degradation by xrn1. May also be involved in post-transcriptional maturation of mRNA poly(A) tails. pan3 acts as a positive regulator for PAN activity, recruiting the catalytic subunit pan2 to mRNA via its interaction with RNA and with pab1. Homodimer. Forms a heterotrimer with a catalytic subunit pan2 to form the poly(A)-nuclease (PAN) deadenylation complex. Interacts (via PAM-2 motif) with poly(A)-binding protein pab1 (via PABC domain), conferring substrate specificity of the enzyme complex. The N-terminal zinc finger binds to poly(A) RNA. Contains a pseudokinase domain. The protein kinase domain is predicted to be catalytically inactive because some of the residues important for catalytic activity are substituted and it lacks the equivalent of the binding site for a peptide substrate. However, it has retained an ATP-binding site and ATP-binding is required for mRNA degradation, stimulating the activity of the pan2 nuclease in vitro. The nucleotide-binding site is juxtaposed to the RNase active site of pan2 in the complex and may actually bind nucleosides of a poly(A) RNA rather than ATP, feeding the poly(A)-tail to the active site of the deadenylase and thus increasing the efficiency with which this distributive enzyme degrades oligo(A) RNAs. The pseudokinase domain, the coiled-coil (CC), and C-terminal knob domain (CK) form a structural unit (PKC) that forms an extensive high-affinity interaction surface for pan2. Belongs to the protein kinase superfamily. PAN3 family.