How thylakoids get their curves

Here’s something new under the sun… specific proteins that help create the curved shape of stacked thylakoid membranes.

The stacked membrane structure of the grana in chloroplasts allows for a large amount of surface area for the photosynthetic light reactions within a small space. How the membranes contort into this shape is not known, but something must perform this function within the chloroplast because the lipids in the membranes do not adopt these structures on their own. In a new paper from Plant Cell, Armbruster and colleagues report the identification of a new protein family called CURVATURE THYLAKOID1 that helps the thylakoids curve themselves into grana stacks. Click here* to read the article yourself.

Here’s how the method breaks down…

Observations: How the extreme membrane curvature of stacked grana membranes is formed and maintained is unknown. There must be some proteins within the thylakoid membranes that perform this function. Proteins that have related functions are often co-regulated at the gene transcription level, so transcriptional data was analyzed to find genes and proteins of unknown function that are probably related to photosynthetic function.

Hypothesis: One protein family identified from this analysis (CURVATURE THYLAKOID1, CURT1) functions to maintain the curved shape of the thylakoid membranes in the grana stacks of chloroplasts.

Experiment: The CURT1 proteins were studied with respect to localization in the thylakoids in normal plants. Researchers also studied mutant plants; some plants lacking CURT1 proteins and other plants with increased levels of the CURT1 proteins. The photosynthetic reactions and thylakoid membrane structure of these mutants were measured. Investigators also assayed the effects of the CURT1 proteins on isolated membrane systems.

Results: The CURT1 proteins were found to be enriched in the grana margin regions of the thylakoids, where the membranes are the most curved. Mutant plants missing the CURT1 proteins had flat thylakoids and few grana stacks, while mutant plants that had more CURT1 proteins had more grana stacks (all relative to normal plants). The CURT1 proteins also induced curved structures in the isolated membrane system.

Conclusion: The CURT1 family is a group of thylakoid membrane proteins that function together to induce the highly curved structure necessary for the shape of the grana stacks in chloroplasts.

Think Ahead: There are four proteins in the CURT1 family and this work suggests they interact with one another. Future studies will focus on how they interact with one another and what the overall structure of these protein complexes is to gain insight into how they create curves in the thylakoids. It will also be interesting to see how these proteins work in the pathway for the longer-term remodeling of thylakoid membrane structure in response to different light conditions.

This figure is an adaptation of part of Fig9 from the paper illustrating the trends in thylakoid membrane structure at the indicated amounts of CURT1 proteins

This paper is a good example of how some projects do not have specific hypotheses at the outset of the investigation. The premise of the experiments that led to this discovery was that genes and proteins related to photosynthetic function would be co-regulated at the level of transcription. Since datasets of this kind already exist and there are numerous known photosynthetic genes and proteins, these data can be mined to find previously unknown (to be associated with photosynthesis) and probably interesting genes and proteins. This was the original study reported by Biehl and co-authors in 2005. Once candidates are identified, mutants lacking these proteins are studied to identify specific defects to pinpoint the roles the newly identified candidates in photosynthesis. In this model, researchers are constantly adjusting their hypothesis based on results until a cohesive story emerges. For these groups, this is the second publication describing the function of newly identified candidate genes in photosynthesis, so the strategy is working out just fine.

Johnna

*Reference: Arbruster et al, Arabidopsis CURVATURE THYLAKOID1 Proteins Modify Thylakoid Architecture by Inducing Membrane Curvature Plant Cell 2013; First Published on July 9, 2013 doi: http:/​/​dx.​doi.​org/​10.​1105/​tpc.​113.​113118 

This reference is behind a paywall** for the journal and requires a subscription to view the full text.

**If you cannot read this article because you do not have access and are somewhat outraged/indignant, then check back in the next week or so when I cover more details about the wonderful world of scientific publishing.

Reference: Biehl et al, Analysis of 101 nuclear transcriptomes reveals 23 distinct regulons and their relationship to metabolism, chromosomal gene distribution and co-ordination of nuclear and plastid gene expression. Gene, Volume 344, 3 January 2005, Pages 33–4. http://dx.doi.org/10.1016/j.gene.2004.09.009