Evolutionary repurposing of trypanosomal Pam18 and Pam16 reveals a new regulatory circuit for mitochondrial genome replication

Protein import and genome replication are essential processes for mitochondrial biogenesis and propagation. The J-domain proteins Pam16 and Pam18 regulate the presequence translocase of the mitochondrial inner membrane. In the protozoan Trypanosoma brucei, their counterparts are TbPam16 and TbPam18, which are essential for the procyclic form of the parasite, though not involved in mitochondrial protein import. Here, we show that during evolution, the two proteins have been repurposed to regulate the replication of maxicircles within the intricate kDNA network, the most complex mitochondrial genome known. TbPam18 and TbPam16 have inactive J-domains suggesting a function independent of heat shock proteins. However, their single transmembrane domain is essential for function. Pulldown of TbPam16 identifies a putative client protein, termed MaRF11, the depletion of which causes the selective loss of maxicircles, akin to the effects observed for TbPam18 and TbPam16. Moreover depletion of the mitochondrial proteasome results in increased levels of MaRF11. Thus, we propose a model for a membrane-bound regulatory circuit that controls maxicircle replication in response to an unknown nuclear signal. This model posits that MaRF11 directly mediates maxicircle replication, that its level is controlled by proteasomal digestion, and that it is protected from degradation by binding to the TbPam18/TbPam16 dimer.