VDAC: the Channel at the Interface Between Mitochondria and the Cytosol
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Molecular and Cellular Biochemistry 256/257: 107–115, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 107 VDAC: The channel at the interface between mitochondria and the cytosol Marco Colombini Department of Biology, University of Maryland, MD, USA Abstract The mitochondrial outer membrane is not just a barrier but a site of regulation of mitochondrial function. The VDAC family of proteins are the major pathways for metabolite flux through the outer membrane. These can be regulated in a variety of ways and the integration of these regulatory inputs allows mitochondrial metabolism to be adjusted to changing cellular conditions. This includes total blockage of the flux of anionic metabolites leading to permeabilization of the outer membrane to small proteins followed by apoptotic cell death. (Mol Cell Biochem 256/257: 107–115, 2004) Key words: mitochondrion, outer membrane, apoptosis, isoforms, metabolism Introduction author’s view, see also ref. [6] for an alternative view). In- formation on VDAC can also be found on the VDAC web Mitochondria live, function, and reproduce in a very rich and page: www.life.umd.edu/vdac. friendly environment, the cytosol of the eukaryotic cell. Just as the plasma membrane is the interface between the inter- stitial space and the cytosol, so the mitochondrial outer The VDAC channels: Conservation and membrane is the interface between the cytosol and the mi- tochondrial spaces. Both separate the cell or the organelle specialization from its environment and both act as selective barriers to the entry and exit of matter. These membranes are also logical VDAC channels (Fig. 1) from a wide variety of sources (plants sites for control. While the plasma membrane has long [7–9], animals [1, 3, 10–12], fungi [4, 13], and protists [14, 1 been known to be the site of action of many chemicals and 15]) have been studied to different levels of detail. The pic- electrophysiological signals, a similar role for the outer mem- ture that is emerging is one of a highly functionally-conserved brane is just beginning to be recognized. One way in which archetypal VDAC and isoforms that retain some of the basic the cell can control mitochondrial function is through con- properties but are different in ways that probably indicate spe- trolling the flux of metabolites. This could obviously influence cialized functions. The conservation is not as evident in the both mitochondrial growth and activity and overall cellular primary sequence as it is in the secondary structure deduced activity dependent on mitochondrial energy transduction and from that sequence [16]. The alternating polar-non-polar mitochondria-derived compounds. The common pathway for the translocation of metabolites through the outer membrane is the VDAC channel [1–3]. This article will examine ways 1VDAC is often referred to as mitochondrial porin implying that it is part of in which VDAC can influence metabolite flux through gating a larger family of channels located at the outer membranes of gram negative and interaction with other proteins and the consequences of bacteria. While the evolutionary origin of VDAC is unclear, experimental evi- changing outer membrane permeability on cell survival. For dence strongly indicates that the structure and properties of VDAC are quite different from the rather varied structures and properties of the family of a review that addresses VDAC’s structure and gating mecha- bacterial channels collectively referred to as porins. Any similarity is likely nism (see ref. [4] and updated information in ref. [5] for this coincidental. Hence the term “mitochondrial porin” is misleading. Address for offprints: M. Colombini, Department of Biology, University of Maryland, College Park, MD 20742, USA (E-mail: [email protected]) 108 cytosol positive loop N-terminal protein binding site? extension 46° protein binding site? intermembrane space Fig. 1. Schematic diagram of VDAC in the mitochondrial outer membrane. The barrel that spans the membrane is composed of 1α helix and 13β strands [5]. The staves tilt at 46° to the axis of the pore [85]. Loop regions between the transmembrane strands are likely the sites of interaction with proteins and other soluble factors. A conserved unusually long loop facing the cytosol is an attractive candidate for a protein binding site. There is also a highly positively charged loop in some forms of VDAC including the mammalian isoforms. While the overall length of VDAC is highly conserved, there are N-terminal ex- tensions in some VDACs. These additions may serve a recognition function or as sites of attachment. pattern of the amino-acid side chains in the primary sequence seem to be present in more complex multicellular organisms. is characteristic of β strands separating the polar environment N. crassa seem to have only one form of VDAC, S. cereviciae within the channel from the non-polar membrane phase. (yeast) has 2 [26], and mice and wheat have 3 [8, 27]. Beyond This pattern in 10-amino-acid stretches, sufficient to span just numbers, the knock out of just one of the 3 VDAC isoforms the membrane, can be easily identified using a simple com- in mice causes serious and surprising effects but no single puter algorithm and a suitable hydrophobicity scale [17]. The isoform is required for cell viability [28]. For example, the resulting ‘beta pattern’ is remarkably conserved even when knock-out of VDAC3 in mice results in male sterility due to the percent homology is below the level of statistical signifi- non-motile sperm [29] while the knock-out of either VDAC1 cance (16). Nevertheless, not all predicted β strands turned or VDAC2 yields mice with a 30% reduction in respiratory out to be transmembrane when tested experimentally. Three capacity [28]. The lack of VDAC1 results in embryonic death lines of experimental evidence [5, 18, 19] points to the con- of some mice between 10.5 and 11.5 days after conception [25]. clusion that the barrel of the channel is formed by 1 α helix More frequent embryonic death occurs in mice lacking both and 13 β strands (Fig. 2.). It is also established that a single VDAC1 and VDAC3 and these mice also show serious growth polypeptide chain forms one channel [20, 21]. retardation. The growth of yeast cells lacking yeast VDAC1 The archetypal VDAC is often labeled as VDAC1. Its fun- and expressing one of the three wheat VDAC isoforms varied damental properties (single-channel conductance, selectivity, depending on the isoform with wheat VDAC3 expressing cells voltage dependence) are highly conserved in all eucaryotic being the slowest growing [9]. The findings did not seems to kingdoms [12]. However, detailed studies have revealed im- be related to expression levels or amount of functional protein portant differences. For example, mammalian VDAC is in the mitochondrion but to the molecular properties of the sensitive to La3+ [22] while N. crassa VDAC is not [23]. isoforms. Results such as these strongly indicate functional Conversely, N. crassa VDAC is modulated by G-actin while specialization rather than redundancy. mammalian VDAC is not [24]. These differences undoubt- Three VDAC-like proteins have been identified in D. mel- edly reflect differences in the regulatory functions within the anogaster [30] (also Brett Graham and William Craigen, different cell types. unpublished). These have sufficient similarity to the canoni- Additional specialization takes the form of VDAC iso- cal VDAC to be recognized but also contain major differ- forms obtained both by the expression of distinct autosomal ences. These are in addition to the clearly recognized D. genes and by alternative splicing [25]. So far more isoforms melanogaster VDAC protein whose properties match those 109 A N K S I YQKVNKKLETAVNL FT K I A T E K K G W L K G T G K A A I G F R T N S N G A T human VDAC1 G W T G F Y Y N N Y E Q E T P K T I AVPP G T D S R G D K T N L W N F E YE D P L P A Y A E E R T S H G G N D T G Q D L S F Y L S I L W V A Q I F G K K T K G D N V L N C T K E S A T S T T F L L A T F Y L L R D K S E E T G A G H S G T G V F L G L I L C G Y L A L L L T K D S S T K D R Q Q K G S G G Y L A G V L M I M F V I A L G F K N T E G D S N E N L L K G I T V D R F P F D N S L H L E K Q A D G E T S D G TT L IA T K G G V R S K A Y K A T G N N QSNFAV V K111 A F G N SYYHKVNSQVEAGSKA N. crassa VDAC R L A T A H S189 W N LT68 N F F K T A V E L G V T N G T K S G V T K V Y N T A Q S R P Q P S L I E252 AVP N37 K T L N N D R G A N V D W F F H E144 D P L V S282 F S P A T N S H G G T V L T E D I T F F T F G V F A S V L F A K N K K A I R I L T F N G T S A S V G N G A A A I V Y V F N D K T E A E F D G A K A G S L L V G L L A F I A A G I A T N K E K A E K D N S S K A S G D F I S G T L L A A Y I A F V Y H T T S K G L T G T N V D K L G H T V K K P Y P D G T H A A D V E A G D A R Q T K M L V H K A A N I A A K Q Y L Q D T G D GYSAAV Fig.