Nature of In-Plane Phase Transitions in 2D Crystals of the Mitochondrial Porin, VDAC

VDAC is a voltage-gated ion and metabolite channel that occurs at high density in the mitochondrial outer membrane. Although VDAC is probably related structurally to bacterial porins, small transmembrane voltages cause it to undergo reversible, partial closures that are not seen with the prokaryotic pores. The “closed” states, which are impermeable to ATP, can be induced by effectors, including a synthetic polyanion. There is evidence that closure involves major rearrangements of the pore structure that are difficult to explain in terms of porin-like β-barrels.The main source of information about the structure of VDAC is electron microscopy of 2D crystals obtained by phospholipase treatment of outer membranes of fungal mitochondria. The unit cell observed after partial lipid hydrolysis (a = 13.3 nm, b = 11.5 nm, γ = 109°) contains six pores which appear to be structurally equivalent at the resolution of correlation averages of crystals embedded in aurothioglucose or vitreous ice (∼1/1.5 nm−1).