Association of mitochondrial fucosyltransferase TbFUT1 with the assembly of the mitochondrial FoF1-ATP synthase in bloodstream form Trypanosoma brucei.

Abstract

The gene TbFUT1 encodes an essential fucosyltransferase which, unexpectedly, localises to the mitochondrion of the protist parasite Trypanosoma brucei. The expression of TbFUT1 is required for the maintenance of mitochondrial membrane potential ({Psi}{Delta}m) in the bloodstream form (BSF) of the parasite, but the precise functions of TbFUT1 are unknown. Here, we demonstrate that depletion of TbFUT1 causes the accumulation of dyskinetoplastid cells; i.e., cells lacking concatenated complexes of mini- and maxicircle kinetoplast DNA (kDNA), the mitochondrial DNA of these organisms. Morphological analysis by serial face block-scanning electron microscopy showed that the dyskinetoplastid mitochondria were otherwise unperturbed with respect to structure and volume. Proteomics analyses showed that TbFUT1 depletion caused a decrease in the steady-state levels of several subunits of the Fo-subcomplex and peripheral stalk components of the mitochondrial FoF1-ATP synthase, as well as a pronounced reduction in mitochondrial ribosomal large subunit (LSU) proteins and more minor reduction in small subunit (SSU) proteins. TbFUT1 was rendered redundant with respect to cell survival and {Psi}{Delta}m generation upon F1-{gamma}WT/L262P mutation; a mutation that allows the generation of {Psi}{Delta}m in the absence of mitochondrial translation. Additionally, depletion of TbFUT1 no longer perturbs kDNA replication in these cells, indicating that dyskinetoplasty is a downstream consequence of impaired {Psi}{Delta}m. Depletion of TbFUT1 in wild type cells leads to the collapse of {Psi}{Delta}m via a functional FoF1-ATP synthase complex. We therefore conclude these mutants are inhibited in the synthesis of Fo-subcomplex components and, thus, impairing the assembly of functional FoF1-ATP synthase complexes. Curiously, mitochondrial transcript levels exhibit similar changes in abundance after FUT1 ablation in the parental and F1-{gamma}WT/L262P mutants. Further, the [~]5-fold overexpression of TbFUT1 in the TbFUT1 conditional knockout mutant under permissive conditions selectively inhibits the formation of the fully RNA-edited A6 transcript by an unknown mechanism, partially suppressing FoF1-ATP synthase assembly in these mutants. Together, these data suggest that mitochondrial fucosylation is essential for the assembly of protein complexes containing kDNA encoded subunits.