| 1.B.28 The Plastid Outer Envelope Porin of 24 kDa (OEP24) Family OEP24 is a high conductance, homooligomeric porin in the outer envelopes of pea chloroplasts, etioplasts and non-green root plastids. It forms a voltage-dependent, high conductance, slightly cationic-selective ion channel in reconstituted proteoliposomes. OEP24 transports triosephosphates, dicarboxylates, positively and negatively charged amino acids, sugars, ATP, and inorganic phosphate and thus exhibits low solute specificity. Structural predictions suggest the presence of seven amphipathic β-strands in a β-barrel arrangement. It contains 213 aas and shows a single homologue in Arabidopsis thaliana (212 aas; 60% identity). OEP24 can functionally replace mitochondrial VDAC (MPP family; TC #1.B.28) when the OEP24 gene is expressed in Saccharomyces cerevisiae. Thus, in yeast, it is targeted to the outer mitochondrial membrane, inserts properly, and functionally replaces VDAC. Nevertheless, it shows no sequence similarity to VDAC. The generalized reaction catalyzed by OEP24 is:
solute (cytoplasm) solute (chloroplast intermembrane space)
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This family belongs to the Outer Membrane Pore-forming Protein III (OMPP-III) Superfamily .
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| References: |
Bölter, B. and J. Soll. (2001). Ion channels in the outer membranes of chloroplasts and mitochondria: open doors or regulated gates? EMBO J. 20: 1-6.
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Klinger, A., V. Gosch, U. Bodensohn, R. Ladig, and E. Schleiff. (2019). The signal distinguishing between targeting of outer membrane β-barrel protein to plastids and mitochondria in plants. Biochim. Biophys. Acta. Mol. Cell Res 1866: 663-672.
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Pohlmeyer, K., J. Soll, R. Grimm, K. Hill, and R. Wagner. (1998). A high-conductance solute channel in the chloroplastic outer envelope from Pea. Plant Cell 10: 1207-1216.
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Röhl T., M. Mitzkus and J. Soll. (1999). The outer envelope protein OEP24 from pea chloroplasts can functionally replace the mitochondrial VDAC in yeast. FEBS Lett. 460: 591-594.
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Ulrich, T., L.E. Gross, M.S. Sommer, E. Schleiff, and D. Rapaport. (2012). Chloroplast β-barrel proteins are assembled into the mitochondrial outer membrane in a process that depends on the TOM and TOB complexes. J. Biol. Chem. 287: 27467-27479.
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| Examples: |
| TC# | Name | Organismal Type | Example |
| 1.B.28.1.1 | High conductance chloroplast outer envelope OEP24 porin (Pohlmeyer et al. 1998). | Plants | OEP24 of Pisum sativum |
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| 1.B.28.1.2 | OEP24 homologue | Plants | OEP24 homologue of Selaginella moellendorffii |
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| 1.B.28.1.3 | OEP24 homologue | Green algae | OEP24 homologue of Chlorella variabilis |
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| 1.B.28.1.4 | Oep24 of 213 aas. High-conductance voltage-dependent solute
channel with a slight selectivity for cations, transporting
triosephosphates, dicarboxylic acids, ATP, inorganic phosphate (Pi),
sugars, and positively or negatively charged amino acids. The central mitochondrial components that mediate the import of yeast
β-barrel proteins can deal with precursors of chloroplast β-barrel
proteins (Ulrich et al. 2012). Targeting and surface recognition of mitochondrial β-barrel proteins in
yeast, humans and plants depends on the hydrophobicity of the last
β-hairpin of the β-barrel, but the presence of a hydrophilic amino acid at the
C-terminus of the penultimate β-strand is also required for mitochondrial
targeting (Klinger et al. 2019). | | Oep24 of Arabidopsis thaliana (Mouse-ear cress) |
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| Examples: |
| TC# | Name | Organismal Type | Example |
| 1.B.28.2.1 | Putative OEP24 homologue of 286 aas | Plants | OEP24 homologue of Coccomyxa subellipsoidea |
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| 1.B.28.2.2 | Uncharacterized protein of 279 aas | | UP of Chlamydomonas sp. UWO 241 |
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| 1.B.28.2.3 | Uncharacterized protein of 223 aas | | UP of Nannochloris sp. 'desiccata' |
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| 1.B.28.2.4 | Uncharacterized protein of 295 aas | | UP of Myrmecia bisecta |
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