Helicase SUV3, polynucleotide phosphorylase, and mitochondrial polyadenylation polymerase form a transient complex to modulate mitochondrial mRNA polyadenylated tail lengths in response to energetic changes.

Mammalian mitochondrial mRNA (mt-mRNA) transcripts are polyadenylated at the 3' end with different lengths. The SUV3·PNPase complex and mtPAP have been shown to degrade and polyadenylate mt mRNA, respectively. How these two opposite actions are coordinated to modulate mt-mRNA poly(A) lengths is of interest to pursue. Here, we demonstrated that a fraction of the SUV3·PNPase complex interacts with mitochondrial polyadenylation polymerase (mtPAP) under low mitochondrial matrix inorganic phosphate (Pi) conditions. In vitro binding experiments using purified proteins suggested that SUV3 binds to mtPAP through the N-terminal region around amino acids 100-104, distinctive from the C-terminal region around amino acids 510-514 of SUV3 for PNPase binding. mtPAP does not interact with PNPase directly, and SUV3 served as a bridge capable of simultaneously binding with mtPAP and PNPase. The complex consists of a SUV3 dimer, a mtPAP dimer, and a PNPase trimer, based on the molecular sizing experiments. Mechanistically, SUV3 provides a robust single strand RNA binding domain to enhance the polyadenylation activity of mtPAP. Furthermore, purified SUV3·PNPase·mtPAP complex is capable of lengthening or shortening the RNA poly(A) tail lengths in low or high Pi/ATP ratios, respectively. Consistently, the poly(A) tail lengths of mt-mRNA transcripts can be lengthened or shortened by altering the mitochondrial matrix Pi levels via selective inhibition of the electron transport chain or ATP synthase, respectively. Taken together, these results suggested that SUV3·PNPase·mtPAP form a transient complex to modulate mt-mRNA poly(A) tail lengths in response to cellular energy changes.