Transport in and out of plastids: does the outer envelope membrane control the flow?

Abstract

This recent work on OEPs is undoubtedly a significant contribution to our changing view of the properties of plastidic porins. However, electrophysiological measurements of the properties of the different OEPs only describe the potential of these channels to regulate solute flow through the outer envelope membrane and do not tell us about the situation in vivo. Is OEP21 actually regulated by the ATP:TPT-substrates ratio, allowing the efflux of triose phosphates under photosynthetic conditions? If on the one hand, the outer envelope membrane is unspecifically permeable to solutes as previously thought, then the metabolite status in the intermembrane space should reflect that of the cytosol. On the other hand, assuming that the outer envelope membrane can act as a permeability barrier for solutes, the metabolic situation in the intermembrane space should reflect that in the stroma rather than that in the cytosol, although the actual metabolite concentrations in the intermembrane space are unknown. Because the TPT mediates a strict 1:1 exchange of metabolites, the sum of the TPT-substrates in both the cytosol and the stroma is kept almost constant. Under photosynthetic conditions, ATP levels are approximately doubled in the stroma, leading to a higher ATP:TPT-substrates ratio in light compared with dark conditions. On the one hand, this should lead to an inward rectification of OEP21 channels and thus to a restriction of triose phosphate export. On the other hand, the export of triose phosphates has to be balanced by the import of inorganic phosphate (which is used for ATP production). Consequently, a restriction of phosphate import by outwardly rectifying OEP21 channels would lead to phosphate-depletion of chloroplasts and inhibition of photosynthesis13xRole of orthophosphate and other factors in the regulation of starch formation in leaves and isolated chloroplasts. Heldt, H.W. et al. Plant Physiol. 1977; 59: 1146–1155Crossref | PubMedSee all References13. On this basis it would appear unlikely that the rectifying properties of OEP21 are modified by the ATP:TPT-substrates ratio in vivo.In conclusion, the potential for the regulation of metabolite fluxes is clearly demonstrated for OEP21, although their contribution to flux regulation in vivo, particularly during transition from light to dark remains speculative. In view of the fact that several OEP channels present in the outer envelope membrane have overlapping and broad specificities (and there are probably more OEP channels than those identified to date), one wonders whether the permeation of a single solute across the outer membrane can be so restricted. It has been shown that the regulation of channel opening and closing could be achieved by voltage-gating. All three OEPs have the highest open probability at membrane potentials about zero and respond to higher potentials by reducing their open probabilities. However, the existence of a potential across the outer membrane is rather unlikely (except for a Donnan potential) thus the observed voltage gating might not be of physiological importance. The regulation of porin function could also be exerted by physiologically relevant factors (i.e. solutes or interacting proteins). Such interactions have been observed for bacterial and mitochondrial porins (e.g. the interaction of the mitochondrial VDAC with kinases and the inner membrane adenylate translocator14xComplexes between porin, hexokinase, mitochondrial creatine kinase and adenylate translocator display properties of the permeability transition pore. Implication for regulation of the permeability transition by the kinases. Beutner, G. et al. Biochim. Biophys. Acta. 1998; 1368: 7–18Crossref | PubMed | Scopus (249)See all References14), but are as yet poorly understood and are currently under investigation.What can be done to assess the control that the outer envelope membrane might exert on metabolic fluxes between chloroplasts and the cytosol, and to bridge the gap between the described biophysical properties of OEPs and their actual role in plant metabolism? One approach could be to identify the putative interacting factors of the OEPs and to analyse whether complexes formed between these factors and the OEPs can regulate permeability properties of individual OEPs. Another approach would be to identify OEP-homologs in Arabidopsis and to screen and characterize knockout-mutants. If the loss of gene function cannot be substituted by other OEPs (or is only partially substituted for), then these mutants should show alterations in plant metabolism (e.g. a reduced export of triose phosphates should result in a perturbation of starch and sucrose metabolism). However, it is more likely that only the simultaneous elimination of several OEPs will lead to clear-cut phenotypes. This could then be the basis to elucidate the regulation that a particular OEP-channel might exert on solute transport across the outer envelope membrane at the biochemical and physiological levels.