The C-terminal alpha-alpha superhelix of Pat is required for mRNA decapping in metazoa

Pat proteins regulate the transition of mRNAs from a state that is translationally active to one that is repressed, committing targeted mRNAs to degradation. Pat proteins contain a conserved N-terminal sequence, a proline-rich region, a Mid domain and a C-terminal domain (Pat-C). We show that Pat-C is essential for the interaction with mRNA decapping factors (i.e. DCP2, EDC4 and LSm1-7), whereas the P-rich region and Mid domain have distinct functions in modulating these interactions. DCP2 and EDC4 binding is enhanced by the P-rich region and does not require LSm1-7. LSm1-7 binding is assisted by the Mid domain and is reduced by the P-rich region. Structural analysis revealed that Pat-C folds into an alpha-alpha superhelix, exposing conserved and basic residues on one side of the domain. This conserved and basic surface is required for RNA, DCP2, EDC4 and LSm1-7 binding. The multiplicity of interactions mediated by Pat-C suggests that certain of these interactions are mutually exclusive and, therefore, that Pat proteins switch decapping partners allowing transitions between sequential steps in the mRNA decapping pathway.