The Edge of Light and Excitement

- proteins in the chloropalst outer envelope membrane

 

FAQs about the chloroplast outer envelope membrane (OM)   Q1. Why is it important to know about the chloroplast OM? A1. Because of its important functions. It plays a role in communication between the chloroplast and cytoplasm, lipid metabolism, and the organelle movement (see the list of proteins in OM below). Defining the properties of OM should help us better understand these biological processes. A2. Because it is homologous to OMs of Gram-negative bacteria and mitochondria. More we know about it, better we understand how these cellular/organellar compartments have evolved.   Q2. WAIT! Isn't the chloroplast OM JUST the remnant of the phagosomal/vacuolar membrane that was used to engulf the cyanobacterial endosymbiont? A. That idea used to be considered as an example to support the idea of "endosymbiotic theory" (cf. Whatley and Whatley 1981). However, biochemical data suggest that the chloroplast OM has more likely derived from the cyanobacterial OM as reviewed by Joyard et al. 1991 and Inoue 2007. This idea was proposed in '80s (e.g., Cavalier-Smith 1982, 1987).

 

Q3. How are proteins in the chloroplast OM identified?

 A. By various assays such as...

 = Import assay using isolated chloroplasts (e.g., Froehlich et al. 2001)

 = GFP fusion assay (e.g., Oikawa et al. 2008)

 = The presence in the fractionated outer envelope (immunoblotting, enzyme assay, etc..)

 

Q4. How many proteins are there in the chloroplast OM?

 A. Count the number of proteins in the following list, which may be expanding.

 

 

Proteins identified or predicted to be in OM of Arabidopsis thaliana chloroplasts

Ref:

Hsu S-C, Inoue K (2009) Two evolutionary conserved essential β-barrel proteins in the chloroplast outer envelope membrane. BioScience Trends 3, 168-178.

Inoue K (2007) Correction. The chloroplast outer envelope membrane: the edge of light and excitement. Journal of Integrative Plant Biology 49, 1100-1111.

AGI#	Name	Size (kD)	Structure**	Envelope proteome	Function	Phenotype of knockout
Protein Import Components and Their Homologs
At3g46740	atToc75-III	75*	B	YES	Preprotein conducting channel	dead (embryo)
At5g19620	atToc75-V /AtOEP80	<80	B	YES	Unknown	dead (embryo)
At4g09080	atToc75-IV	44	B		Unknown	ok
At4g02510	atToc159	161	C?	YES	Preprotein recognition/translocation	dead (albino seedling)
At2g16640	atToc132	132	C?			ok
At3g16620	atToc120	120	C?			ok
At5g20300	atToc90	88	C?			ok
At1g02280	atToc33	33	C	YES	Preprotein recognition/translocation	not good (pale seedling)
At5g05000	atToc34	35	C	YES		ok
At3g17970	atToc64-III	64	N or P		Preprotein recognition/translocation	ok
At1g80920	Toc12h (pea)	18	X		Preprotein translocation
Putative Solute Channels
At2g28900	OEP16-1	16	P	YES	Cation-selective channel Amino acid transport? Import of PORA precursor?	ok; dead under light after grown in the dark for too long
At4g16160	OEP16-2	19	P			ok
At3g62880	OEP16-4	14	P
At1g20816	OEP21-1	20	B		ATP-regulated anion selective channel
At1g76405	OEP21-2	20	B	YES
At1g45170	OEP24-1	24	B		Cation-selective channel General pore?
At5g42960	OEP24-2	24	B	YES
At2g43950	OEP37	32	B	YES	Cation-selective channel	ok
Lipid Metabolism
At1g77590	LACS9	76	X	YES	Fatty acid activation	ok
At3g11670	DGD1	92	P		Galactolipid biosynthesis	not good
At4g00550	DGD2	54	X		Galactolipid biosynthesis	ok
At5g20410	MGD2	53	X		Galactolipid biosynthesis	ok
At2g11810	MGD3	53	X		Galactolipid biosynthesis	ok
At4g15440	HPLh (tomato)	43	X	YES	Oxylipin metabolism
Others
At5g53280	PDV1	31	C?		Division	fewer larger chloroplasts
At2g16070	PDV2	34	C?		Division	fewer larger chloroplasts
At2g20890	THF1	27*	P?	YES	Sugar signaling; thylakoid formation	variegation
At3g52420	OEP7	6.8	N		Unknown
At5g23190	CYP86B1	64	P		Monooxygenase/suberin synthesis	ok
At3g52230	OMP24h (spinach)	16	C?	YES	Unknown
At5g51020	CRL	30	N or C		Division	not good (pale green, reduced stature, crumpled laminas and irregular leaf margins)
At3g06510	SFR2	70	N or C	YES	β-Glucosidase, freezing tolerance	sensitive to freezing
At3g25690	CHUP1	112	X		Actin-binding, organelle movement	ok

 

Proteins not included in Inoue 2007.

Apparent homologs to known OM proteins from other plant species.

Found in endoplasmic reticulum.

*These two proteins have been shown to be synthesized as larger precursors and processed to be mature forms of the indicated sizes.

**Proteins predicted to be embedded in the lipid bilayer via a single (N: signal-anchor = locating its N-terminus in the intermembrane space; or C: tail-anchor = locating its N-terminus in the cytoplasm), or multiple (P = polytopic) α-helices, β-strands (B) are indicated. Those without any obvious transmembrane domains are indicated as X.

 

REMAINING QUESTIONS:

1. Is this the complete list?

2. How are proteins targeted to OM?

3. Is the "OM proteome" conserved among different plastid types?

4. Do post-translational regulations (e.g., phosphorylation) play any roles in functions of OM proteins?

5. Do OM proteins form oligomeric complexes?

 

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Last updated on November 18th, 2009

 

Kentaro Inoue(C) 2009