1.B.30 The Plastid Outer Envelope Porin of 16 kDa (OEP16) Family
OEP16 is a pea chloroplast cation-selective, highly conductive outer membrane porin with a strong bias for amino acids and primary amines (Pohlmeyer et al., 1997; Samol et al., 2011). It is voltage-sensitive with highest open probability at 0 mV, decreasing exponentially with higher potentials (Bölter and Soll, 2001). It apparently functions as the translocation (import) pore for NADPH:protochlorophyllide oxidoreductase A (PORA) (Samol et al. 2011). Triosephosphates and uncharged sugars are not transported. It appears to be a homooligomer in the membrane. Dimers, trimers and hexamers have been suggested by different groups. It consists of 146 aas and has homologues of about the same size in other plants. A. thaliana has at least three OEP16 paralogues. An N-terminal 60 residue segment exhibits about 35% identity with a C-terminal region of a 251 residue protein from Methanococcus jannaschii (MJ1614; pirE64501). This region (residues 21-93 in OEP16 have been suggested to include two β-strands and one α-helix) that appears to form a channel in liposomes (Steinkamp et al., 2000).
OEP16 shows a large segment that is similar to mitochondrial translocase subunits, Tim 17, Tim 22 and Tim 23 (TC# 3.A.8) as well as the peroxysomal membrane protein 4 (PxMP4; TC# 1.B.69) and a subunit of eukaryotic NADH dehydrogenase complex (TC# 3.D.1). These proteins comprise the Tim17 superfamily (see the TC superfamily link.) OEP16 is predicted to contain both α- and β-structure. The N-terminal domain consists of up to four β-strands while the C-terminal region forms 4 transmembrane α-helices that presumably form the homooligomeric pores. OEP16-1 of A. thaliana is involved in PORA precursor import and by virtue of this activity, confers photoprotection onto etiolated seedlings during greening (Samol et al., 2011). The 3-d NMR structure of OEP16 has been determined (Zook et al. 2013). It consists of an N-termina β-sheet and four C-terminal transmembrane α-helices with TMSs 1 and 2 forming a structure similar to TMSs 3 and 4 (Zook et al. 2013).
The generalized transport reaction catalyzed by OEP16 is:
Cationic solutes and amino acids (cytoplasm) cationic solutes and amino acids (chloroplast).