Governing Respiration: Tubulin's C-Terminus Interaction with VDAC

Abstract

Mitochondria have long been known to localize within and move along the tubulin-microtubule network. It was shown that tubulin binds to isolated mitochondria with high-affinity and specifically associates with the mitochondrial voltage-dependent anion channel (VDAC). We found that nanomolar concentrations of dimeric tubulin vastly increase VDAC sensitivity to voltage allowing for VDAC blockage at low transmembrane potentials. Tubulin interaction with VDAC requires the presence of anionic C-terminal tails (CTT) on the intact protein. Tubulin with proteolytically cleaved CTTs does not block the channel. Actin, also an acidic protein but lacking CTTs, does not induce VDAC blockage. Two synthetic peptides with the sequences of mammalian α and β brain tubulin CTT do not induce detectable channel closure up to micromolar concentrations. These results suggest a completely new role for dimeric tubulin and its charged CTT. We propose a model for tubulin-VDAC channel interaction in which the tubulin CTT penetrates into the channel lumen, potentially reaching through the channel and interacting with a positively charged domain of VDAC. We found that tubulin/VDAC interaction is greatly dependent on the state of VDAC phosphorylation. Remarkably, phosphorylated VDAC is more than an order of magnitude more sensitive to tubulin-induced closure than dephosphorylated VDAC.Tubulin addition to isolated mitochondria increases Km for ADP and lowers oxygen consumption, most likely by restricting flux of ADP through VDAC. Thus interaction of tubulin CTT with VDAC blocks nucleotide passage into and out of mitochondria, thereby regulating oxidative phosphorylation. Examination of the evolution of CTT sequences in mitochondria-containing cells reveals high conservation of charge and length of the tails. Considering the known conservation of VDAC folding pattern throughout mitochondria-containing eukaryotes, we propose that this interaction is widespread and ancient.