1.B.47 The Plastid Outer Envelope Porin of 37 kDa (OEP37) Family
The chloroplast outer envelope protein OEP37 is a β-barrel protein of the outer chloroplast membrane. The reconstituted recombinant protein OEP37 from pea forms a rectifying high conductance channel with a main conductance of 500 picosiemens (symmetrical 250 mm KCl). The OEP37 channel is cation-selective (PK+/PCl- = 14:1) with a voltage-dependent open probability maximum at Vmem = 0 mV. The channel pore reveals an hourglass-shaped form with different diameters for the vestibule and restriction zone. The diameters of the vestibule at the high conductance side were estimated as d = 3.0 nm and the restriction zone as d = 1.5 nm. The OEP37 channel displayed a nanomolar affinity for the precursor of the chloroplast inner membrane protein Tic32, which is imported into the chloroplast through an unknown pathway. Pre-proteins imported through the usual Toc pathway and synthetic control peptides, however, did not show a comparable block of the OEP37 channel.
In addition to the electrophysiological characterization, gene expression of OEP37 in Arabidopsis thaliana was studied. Transcripts of AtOEP37 were ubiquitously expressed throughout plant development and accumulated in early germinating seedlings as well as in late embryogenesis. The plastid intrinsic protein could be detected in isolated chloroplasts of cotyledons and rosette leaves. However, the knock-out mutant oep37-1 shows that the proper function of this single copy gene is not essential for development of the mature plant. Moreover, import of Tic32 into chloroplasts of oep37-1 was not impaired when compared with wild type. Thus, OEP37 may constitute a novel peptide-sensitive ion channel in the outer envelope of plastids functioning during embryogenesis and germination.
Oep37 shows up to twenty potential TM β-strands. They occur throughout its length. Two homologues are found in rice.
The generalized reaction catalyzed by Oep37 is:
Cations (in) Cations (out)