Hidden keyholders – exploring metabolite transport across the outer chloroplast membrane

Figures & data

Figure 1. OEP16, OEP21, OEP23, OEP24, OEP37, OEP40, and JASSY are described as metabolite channels in the OE. Structural predictions were obtained by AlphaFold Jumper et al. (2021; Varadi et al. 2022). Experimental evidence for the topology are only provided for OEP21 and OEP24 (Gross et al. 2021; Gunsel et al. 2023). 6GP = glucose 6-phosphate, question marks represent yet unidentified metabolites.

Table 1. Overview of OE metabolite transporting proteins.

Protein name	(potential) Function	Locus	References	Structure
OEP24A OEP24B	Solute/ion channel Solute/ion channel	At1g45170 At5g42960	Pohlmeyer et al. (1998), Rohl et al. (1999)	β-barrel
OEP21A OEP21B	Solute/ion channel Solute/ion channel	At1g20816 At1g76405	Bolter et al. (1999), Hemmler et al. (2006); Gunsel et al. (2023)	β-barrel
OEP40	Glucose channel	At3g57990	Harsman et al. (2016)	β-barrel
OEP37	Solute/ion Channel	At2g43950	Goetze et al. (2006)	β-barrel
OEP16–1	Amino Acid Channel	At2g28900	Pohlmeyer et al. (1997), Steinkamp et al. (2000), Linke et al. (2004), Philippar et al. (2007), Pudelski et al. (2012), Zook et al. (2013)	α-helical
OEP16–2	Amino Acid Channel	At4g16160	Pohlmeyer et al. (1997), Steinkamp et al. (2000), Linke et al. (2004), Philippar et al. (2007), Pudelski et al. (2012), Zook et al. (2013)	α-helical
OEP16–4	Amino Acid Channel	At3g62880	Pohlmeyer et al. (1997), Steinkamp et al. (2000), Linke et al. (2004), Philippar et al. (2007), Pudelski et al. (2012), Zook et al. (2013)	α-helical
OEP23	Solute/ion Channel	At2g17695	Goetze et al. (2015)	Mixed
JASSY-1	OPDA Channel	At1g70480	Guan et al. (2019)	Mixed
JASSY-2	Unknown/OPDA Channel	At1g23560	Guan et al. (2019)	Mixed

Figure 2. Schematic overview of β-barrel insertion in membranes. (a) insertion into bacterial membranes is facilitated by the SAM complex. Two insertion mechanisms are proposed: 1) an intermediate pore of the Sam50 and the new β-barrel is formed by consecutive subsitution of β-sheets. 2) the new β-barrel is folded in the periplasm and BamA assists the insertion by destabilising the membrane. (b) β-barrel insertion in mitochondria requires the prior translocation via the TOM complex. β-barrels are bound by small TIM proteins in the IMS and inserted by the opening of a lateral gate through Sam50. (c) β-barrel insertion in chloroplasts requires translocation by Toc75. OEP80 is involved in membrane insertion, but mechanistic details are lacking. For clarity only the core proteins of the BAM, SAM, TOM and TOC complexes are shown.

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Data availability statement

Data sharing is not applicable to this article as no new data were created or analysed in this study.