Article

Atomic-Level Models of the Bacterial Carboxysome Shell

Department of Chemistry and Biochemistry, University of California at Los Angeles (UCLA), Los Angeles, CA 90095, USA.
Science (Impact Factor: 31.48). 03/2008; 319(5866):1083-6. DOI: 10.1126/science.1151458
Source: PubMed

ABSTRACT The carboxysome is a bacterial microcompartment that functions as a simple organelle by sequestering enzymes involved in carbon fixation. The carboxysome shell is roughly 800 to 1400 angstroms in diameter and is assembled from several thousand protein subunits. Previous studies have revealed the three-dimensional structures of hexameric carboxysome shell proteins, which self-assemble into molecular layers that most likely constitute the facets of the polyhedral shell. Here, we report the three-dimensional structures of two proteins of previously unknown function, CcmL and OrfA (or CsoS4A), from the two known classes of carboxysomes, at resolutions of 2.4 and 2.15 angstroms. Both proteins assemble to form pentameric structures whose size and shape are compatible with formation of vertices in an icosahedral shell. Combining these pentamers with the hexamers previously elucidated gives two plausible, preliminary atomic models for the carboxysome shell.

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    • "Tsai et al. (2007) 15 Proteins CsoS4A from Halothiobacillus neopolitanus and CcmL from Synechocystis 6,803 were reported to crystallize as pentamers and form vertices of carboxysome shell (Tanaka et al. 2008). Crystal structure of shell proteins led to a proposed model for carboxysome shell (Tanaka et al. 2008) The evolutionary analysis of different forms of RuBisCO revealed Form III RuBisCO to be source of all RuBisCO and RLP lineages (Tabita et al. 2008) 2008 Tanaka et al. (2008); Tabita et al. (2008) The b proteobacteria are generally less flexible in their metabolic activities as compared to a proteobacteria. The forms of RuBisCO found in various members of the group are form II, IC, IAq and IAc. "
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    • "It is plausible that RuBisCO seeds assembled de novo may be brought into close proximity with preexisting carboxysomes by other mechanisms. The carboxysome itself may be sufficient to capture independent seeds: crystal packing evidence from other studies suggests that shell proteins may contact one another face-to-face or assemble into antiparallel strips [11,24]. This may expose cargo-interacting surfaces to the outside of the carboxysome, creating affinity for cargo on the exterior as well as the interior carboxysome surface (Figure 6E). "
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