Regulation of mitochondrial outer membrane (MOM) permeability has dual importance: in normal metabolite and energy exchange between mitochondria and cytoplasm and thus in control of respiration, and in apoptosis by release of apoptogenic factors into the cytosol. However, the mechanism of this regulation, dependent on the voltage-dependent anion channel (VDAC), the major channel of MOM, remains controversial. A long-standing puzzle is that in permeabilized cells, adenine nucleotide translocase (ANT) is less accessible to cytosolic ADP than in isolated mitochondria. We solve this puzzle by finding a missing player in the regulation of MOM permeability: the cytoskeletal protein tubulin. We show that nanomolar concentrations of dimeric tubulin induce voltage-sensitive reversible closure of VDAC reconstituted into planar phospholipid membranes. Tubulin strikingly increases VDAC voltage sensitivity and at physiological salt conditions could induce VDAC closure at <10 mV transmembrane potentials. Experiments with isolated mitochondria confirm these findings. Tubulin added to isolated mitochondria decreases ADP availability to ANT, partially restoring the low MOM permeability (high apparent K m for ADP) found in permeabilized cells. Our findings suggest a previously unknown mechanism of regulation of mitochondrial energetics, governed by VDAC and tubulin at the mitochondriacytosol interface. This tubulin-VDAC interaction requires tubulin anionic C-terminal tail (CTT) peptides. The significance of this interaction may be reflected in the evolutionary conservation of length and anionic charge in CTT throughout eukaryotes, despite wide changes in the exact sequence. Additionally, tubulins that have lost significant length or anionic character are only found in cells that do not have mitochondria.xidative phosphorylation requires transport of metabolites, including cytosolic ADP, ATP, and inorganic phosphate, across both mitochondrial membranes for F 1 F 0 -ATPase to generate ATP in the matrix. Voltage-dependent anion channel (VDAC, also called mitochondrial porin) is the most abundant protein in mitochondrial outer membrane (MOM) and is known to be primarily responsible for ATP/ADP flux across the outer membrane (1, 2). Until recently, VDAC was generally viewed as a part of the pathway for release of cytochrome c and other apoptogenic factors from the mitochondrial intermembrane space into the cytosol at the early stage of apoptosis. The recent genetic studies undermined this view (3) but still left open a lot of questions concerning the role of VDAC in MOM permeabilization in apoptosis (4-6). A conserved property of VDACs in vitro is the ability to adopt a unique fully open state and multiple states with significantly smaller conductance (7). It was demonstrated that the latter, so called ''closed states'' are impermeable to ATP but still permeable to small ions (8), including Ca 2ϩ (9). In isolated mitochondria, respiration is characterized by an apparent K m for exogenous ADP that is Ϸ10-fold lower than in permeabilize...

Critical to biological processes such as secretion and transport, protein-lipid interactions within the membrane and at the membrane-water interface still raise many questions. Here we examine the role of lipid headgroups in these interactions by using gramicidin A (gA) channels in planar bilayers as a probe. We show that although headgroup demethylation from phosphatidylcholine (DOPC) to phosphatidylethanolamine decreases the lifetime of gA channels by an order of magnitude in accordance with the currently accepted hydrophobic mismatch mechanism, our findings with diether-DOPC suggest the importance of the headgrouppeptide interactions. According to our x-ray diffraction measurements, this lipid has the same hydrophobic thickness as DOPC but increases gA lifetime by a factor of 2. Thus we demonstrate that peptide-headgroup interactions may dominate over the effect of hydrophobic mismatch in regulating protein function.