Channel-formers in the mitochondrial outer membrane: Functions and properties

Mitochondria play a very important role in programmed cell death (apoptosis). When growth factor is removed from growth factor-dependent cells, mitochondrial outer membrane (MOM) permeability to anionic metabolites is impaired, followed by cytochrome-c release several hours later. Applying Bcl-x_L to these isolated mitochondria restores MOM permeability without inducing cytochrome-c release. Bcl-x_L also restores MOM permeability to ADP induced by addition of exogenous β-NADH to normal mitochondria. β-NADH limits ADP permeation through the MOM by closing VDAC. Bcl-x_L itself cannot restore MOM permeability, but it restores the permeability by opening VDAC. Together our results provide a possible mechanism by which Bcl-x_L prevents apoptosis at an early stage.; Unlike Bcl-x_L, avicin G, a triterpenoid saponin, induces cytochrome-c release from mitochondria. Avicin G forms two populations of cation-selective channels: one is 0.8 nS and the other one is 0.3 nS in 0.1 M KCl in phospholipid membranes containing cholesterol. Without cholesterol, avicin G only forms 0.3 nS channels under the same conditions. Avicin G causes a much higher total conductance in cholesterol-containing membranes than in cholesterol-free membranes. When the only acyl chain of avicin G is truncated, avicin G loses the channel-forming ability, indicating the acyl chain may be the transmembrane domain. Avicin G channels are composed of four monomers and show a simple ohmic relationship between current and voltage. Our results provide insight into a possible mechanism underlying the pro-apoptotic function of avicin G and help explain other pharmacological functions, such as the hemolytic function, of other triterpenoid saponins.; Bcl-x_L, VDAC and avicin G must encounter a large energy barrier when inserting into membranes. “Auto-directed insertion” is a process that characterizes VDAC insertion. This mechanism must be specific to avoid disastrous outcomes. Results in Chapter 3 show that only PorA/C1 and gramicidin accelerate insertion of VDAC, but α-hemolysin and KcsA do not. VDAC accelerates insertion of PorA/C1 and KcsA, but not that of other proteins. Our results indicate that the accelerated insertion mechanism shows a very high degree of specificity and may help improve the fidelity and efficiency of protein insertion into intracellular membranes.