Sorgato M C, Moran O
Dipartimento di Chimica Biologica, Università di Padova, Italy.
Crit Rev Biochem Mol Biol. 1993;28(2):127-71. doi: 10.3109/10409239309086793.
Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.
亲水分子在线粒体外膜上的快速扩散与一种29至37 kDa的蛋白质有关，这种蛋白质被称为电压依赖性阴离子通道（VDAC），当整合到平面脂质双分子层中时能够产生大的水通道。来自不同来源的VDAC的功能特性在人工膜中似乎高度保守：在低跨膜电位下，通道处于高导通状态，但电位升高（正电位和负电位）会大幅降低电流，并将离子选择性从轻微阴离子型转变为阳离子型。因此有人提出，VDAC不仅仅是一个分子筛，它可能通过响应电压限制不同价态代谢物的进入和/或通过与膜间隙中的可溶性蛋白质相互作用来控制线粒体生理功能。然而，膜片钳和尖端浸入技术的最新应用表明，外膜的电行为不同，其他蛋白质可能在分子渗透中起作用。对VDAC的生化研究、定点突变体的使用以及二维晶体阵列的电子显微镜观察，都有助于提出单体β桶作为通道的结构模型。通过膜片钳直接观察膜，对哺乳动物线粒体内膜的生理学有了重要的认识。在单通道水平上，107 pS的轻微阴离子型、电压依赖性电导和9.7 pS的K⁺选择性且对ATP敏感的电导是最具特征的。然而，在某些条件下，内膜也可能显示非选择性的nS峰值转变，这可能是由多种底物的协同组装引起的。
