Structural basis for P an3 binding to P an2 and its function in mRNA recruitment and deadenylation

Abstract

The conserved eukaryotic P an2– P an3 deadenylation complex shortens cytoplasmic mRNA 3′ polyA tails to regulate mRNA stability. Although the exonuclease activity resides in Pan2, efficient deadenylation requires P an3. The mechanistic role of P an3 is unclear. Here, we show that P an3 binds RNA directly both through its pseudokinase/ C ‐terminal domain and via an N ‐terminal zinc finger that binds polyA RNA specifically. In contrast, isolated Pan2 is unable to bind RNA . Pan3 binds to the region of Pan2 that links its N‐terminal WD 40 domain to the C‐terminal part that contains the exonuclease, with a 2:1 stoichiometry. The crystal structure of the Pan2 linker region bound to a Pan3 homodimer shows how the unusual structural asymmetry of the Pan3 dimer is used to form an extensive high‐affinity interaction. This binding allows Pan3 to supply Pan2 with substrate polyA RNA , facilitating efficient mRNA deadenylation by the intact Pan2–Pan3 complex. image Regulation of mRNA poly(A) tails constitutes an important layer of mRNA stability and translation control. Structural and biochemical studies of the conserved Pan2–Pan3 deadenylation complex now reveal how the non‐catalytic subunit Pan3 contributes to coordinate activities within the complex, and thus to remove poly(A) tails. Pan3 binds to poly(A) RNA through an N‐terminal zinc finger and its pseudokinase/C‐terminal domain. The exonuclease subunit Pan2 does not bind RNA by itself. A single Pan2 protein binds to an asymmetric Pan3 homodimer via an extensive, high‐affinity interaction. Pan3 functions to supply the exonuclease Pan2 with substrate poly(A) RNA , facilitating efficient mRNA deadenylation.