Voltage-Dependent Channels Found in the Membrane Fraction of Corn Mitochondria

Abstract

Transmembrane channels have been found in the membrane fraction of corn (Zea mays W64AN) mitochondria that exhibit a remarkable resemblance to the voltage dependent anion-selective channels (VDAC) located in the outer membrane of animal (Rattus norvegicus), protist (Paramecium aurelia), and fungal (Neurospora crassa) mitochondria.The channels in corn were demonstrated to be essentially identical to VDAC channels in three characteristic properties: (a) single channel conductance magnitude, (b) weak anion selectivity, and (c) nature of voltage dependence. These findings led us to conclude that the channels present in corn mitochondria are VDAC channels. This discovery may have repercussions concerning the regulation and function of higher plant mitochondria, and the causation of higher plant excitability.Mitochondria have an inner and an outer membrane. The inner membrane is known to be involved (among other functions) in the processes of cellular respiration, and the generation of ATP. In contrast, the exact function of the outer membrane is not as well understood. Early research on the outer membrane of isolated mitochondria indicated that the outer membrane is freely permeable to molecules as large as several thousand daltons (17,19, 26, 27). In animals (Rattus norvegicus), protists (Paramecium aurelia), and fungi (Neurospora crassa), the permeability ofthe outer mitochondrial membrane has been demonstrated to be due to large transmembrane protein channels, which form 3 to 4 nm in diameter aqueous pathways through the membrane (2,10,12,24). These channels are distinguished from all other channels by their location in the outer membrane of mitochondria, and by three characteristic properties: (a) the magnitude of their single channel conductance, (b) their weak anion selectivity, and (c) the nature of their voltage dependence (4,11,24 (2,4,7,23,24). They are referred to as being voltage dependent because the probability of which state the channels will be in at a given instant in time is a function of the voltage difference across the membrane containing them (2,4,7,23,24). The VDAC channels are also anion selective since they preferentially allow anions to pass through them as opposed to comparably sized and charged cations (3,24). 'Supported by National Institutes of Health grant No. GM28450.2 Abbreviation: VDAC, voltage dependent anion-selective channels.In higher plants, negatively stained outer mitochondrial membranes have been examined by EM. Points of stain with average center to center spacings of 4.5 nm were observed (18). Channels have been extracted from mung bean mitochondrial outer membranes whose pore size is very similar to VDAC channels (28). These findings suggest that there are large transmembrane channels in the outer membrane of higher plant mitochondria. A study of these channels could prove to be quite interesting since all mitochondria examined to date have been shown to possess VDAC channels (2,4,7,23,24). Thus, any observed differences in properties between the channels in hig...

Purification and Characterization of the Voltage-Dependent Anion-Selective Channel Protein from Wheat Mitochondrial Membranes

“…The channel reopened (downward deflection) as the applied voltage returned toward zero. Thus, as is the case for VDACs from other sources such as R. norvegicus (Colombini, 1979), Z. mays (Smack and Colombini, 1985), or S. cerevisiae (Forte et al, 1987), MmP29 produces a channel that closes at both positive and negative potentials.…”

“…Channels with properties essentially indistinguishable from those of VDAC channels from other sources such as Paramecium aurelia (Schein et al, 1976), Neurospora crassa (Freitag et al, 1982), Rattus norvegicus liver (Colombini, 1983), Zea mays root (Smack and Colombini, 1985), or Saccharomyces cerevisiae (Forte et al, 1987) were observed when MmP29 was reconstituted into planar phospholipid membranes. Like other VDAC channels, MmP29 inserted spontaneously when 10 to 20 pL of a Triton X-100-solubilized sample (1% v/v) was added to the aqueous solution (about 4 mL) on one side of the planar membrane.…”

“…Evidence for the existence of pores in the outer membrane of plant mitochondria was reported (Parsons et al, 1965), 6ut plant mitochondrial VDAC was partially characterized only in 1980 (Zalman et al, 1980), when a molecular size-dependent leakage was found through channels formed in the phospholipid membrane of vesicles reconstituted with an enriched preparation of the major outer mitochondrial protein of mung bean (Phaseolus aurcus). A further characterization of mitochondrial membrane preparation from corn (Zea mays) seedlings exhibiting VDAC properties was reported by Smack and Colombini (1985).…”

“…The switching region location is very similar to those of other VDACs studied so far (Table 11). These estimates were made on recordings in which the voltage was being decreased in absolute values (channel opening) to avoid the kinetic delays inherent in the much slower closure process, also found in other VDAC channels being tested (Schein et al, 1976;Colombini, 1979;Smack and Colombini, 1985;Colombini et al, 1987).…”

Direct Measurement of K+ Channels in Thylakoid Membranes by Incorporation of Vesicles into Planar Lipid Bilayers

“…Until now, however, the only plant channels studied using this technique were plant mitochondrial "VDAC" channels (28) and a Ca2+ channel from giant-celled algae (9). In this work, thylakoid vesicles were fused into planar lipid bilayers (17), and single channel events were observed.…”