N-Terminus of Cid14 Activates RNA Unwinding by Mtr4 in the <i>Schizosaccharomyces pombe</i> TRAMP Complex

The TRAMP (<u>Tr</u>f4-<u>A</u>ir2-<u>M</u>tr4 <u>p</u>olyadenylation) complex is a multiprotein assembly that targets RNA substrates for processing or degradation by the nuclear exosome. In <i>Saccharomyces cerevisiae</i>, the helicase activity of Mtr4 is enhanced by TRAMP assembly. Here we identify unexpected species-specific differences in the ability of Mtr4 to unwind a model RNA substrate. Unlike <i>S. cerevisiae</i> Mtr4, <i>Schizosaccharomyces pombe</i> Mtr4 retains RNA-stimulated ATPase activity, but is unable to unwind a model RNA substrate. This decoupling of ATPase and helicase activity is overcome by TRAMP formation. We further demonstrate that activation of helicase activity is accomplished by unique interactions with multiple regions of the intrinsically disordered N-terminus of the poly(A) polymerase, Cid14 (the <i>S. pombe</i> homologue of <i>S. cerevisiae</i> Trf4). Finally, we propose a model where Mtr4 adaptor complexes regulate unwinding activity by coordinating interdomain interactions within the helicase core.