Characterization of the Selective Permeability of Carboxysome Shell Proteins and Implications for CO2 Fixation Efficiency
Plant Biochemistry and Biophysics
Final Report Abstract
The carboxysome is an organelle involved in photosynthesis, which is found in autotrophic bacteria. The organelle is surrounded by a porous, proteinaceous shell, functioning as a selectively permeable membrane. The focus of this study were the protein constituents of the shell and their impact on the organelle’s permeability profile. In a comparative bio-informatic study of 227 genomes encoding for beta carboxysomes, I detected the minimal set of genes that is required to form the beta carboxysome shell, and I found two new classes of carboxysome shell proteins that exist in some species with beta carboxysomes. I also studied expression patterns of beta carboxysome genes in correlation to their genomic locus and found that genes that cluster in a main carboxysome locus are co-expressed, while genes on remote loci are regulated independently. In a second study, I found that the carboxysome shell proteins, which had previously been shown to form homo-oligomers that assemble into the shell, can also form hetero-oligomers. Specifically, the proteins CcmK3 and CcmK4 form heterohexamers when they are heterologously co-expressed. In the center of these heterohexamers is a small pore with predicted biochemical properties that are unlike those of pores formed by CcmK4 homooligomers. Furthermore, the CcmK3-CcmK4 heterohexamers are unlikely to be embedded into the carboxysome shell based on their structure, but they might be able to anneal to extant carboxysome pores, effectively capping them, to transiently alter the permeability of the shell. My findings advance the understanding of how the carboxysome shell is constructed and how its permeability could be fine-tuned. Fine-tuning the shell’s permeability may be particularly important for beta carboxysomes, as their cyanobacterial hosts occupy niches where light and CO2, the resources for photosynthesis, are fluctuating. The results will thus be of fundamental interest for the cyanobacterial research community, ecologists, as well as plant biologists. Moreover, hetero-complex formation by BMC shell proteins provides a powerful new strategy to design tailor-made organelles with dynamic permeability profiles. Initially, I intended to focus the project on characterizing the physiological phenotype of the ccmK3 and ccmK4 deletion mutants. However, when I detected a slow-growth phenotype in the ccmK4 mutant that had previously been described as wildtype-like, I adjusted the priority of the project towards characterizing the biochemical function of CcmK3 and CcmK4. Part of the project was already published under the title "ß-Carboxysome bioinformatics: identification and evolution of new bacterial microcompartment protein gene classes and core locus constraints" in the Journal of Experimental Botany. An online article from the Michigan State University regarding this manuscript titled “How to build an artificial nano-factory to power our futures” was published recently.
Publications
- (2017) β-Carboxysome bioinformatics: identification and evolution of new bacterial microcompartment protein gene classes and core locus constraints. J Exp Bot erx115
Sommer M, Cai F, Melnicki M, Kerfeld CA
(See online at https://doi.org/10.1093/jxb/erx115)
