In the CsoS1D trimers, conformational changes in the absolutely c

In the CsoS1D trimers, conformational changes in the absolutely conserved pore loop residues Glu120 and Arg121 (Fig. 9) result in either a relatively large open pore of ~14 Å diameter or an occluded pore (Fig. 10). The large size of the CsoS1D pore, which would allow for free passage of RuBP, likely requires gating

to prevent the loss of important metabolites or infiltration of inhibitory species. Fig. 10 Electrostatic comparison of the two trimers of the tandem BMC-domain protein CsoS1D (PDB:3F56) and modeled representation of the “air-lock” mechanism for metabolite movement through the protein. Convex (top), concave (middle), and pore cross-section (bottom) views are shown for each of the two structures on the left. The top and bottom BMN 673 nmr images of the “air-lock” mechanism are generated from the same solved stacked structure from two different orientations. The middle

image is a hypothetical model generated in PyMOL by structurally aligning a copy of a closed trimer over the open trimer in the stacked structure. Red denotes negative charge and blue denotes positive charge Interestingly, in two independent crystal structures, the CsoS1D trimers stacked to form a dimer of trimers (Fig. 10). The two trimers were rotated ~60° with respect to each LCZ696 nmr other so that the C-terminal domain of a subunit in the upper trimer interacted with the N-terminal domain of a subunit in the lower trimer. The dimerization was across the concave face of each trimer, resulting in a large cavity of 13,613 Å3. Additional biophysical analyses that support the potential biological relevance for the dimer of trimers include a buried surface area of 6,573 Å2 and a shape correlation value of 0.70 (range of 0–1, 1 being a perfect fit and 0 being no interaction) between the two trimers

(Klein et al. 2009). The cavity could, like the pore gating, influence the flux of larger metabolites (e.g., RuBP, 3PGA) into and out of the carboxysome in a manner analogous to an airlock. For example, the trimer facing the cytosol would open to accept a metabolite and then close; subsequently, the trimer facing the carboxysome interior would open to allow for release of the metabolite from the cavity (Fig. 10). An ortholog to CsoS1D, with the locus tag slr0169 in Synechocystis sp. PCC6803, has also been identified in all β-carboxysome-containing cyanobacteria (Klein et al. 2009). It is ~200 amino acids in length and lacks ~50 N-terminal residues that are present in the α-cyanobacterial CsoS1D homologs. slr0169 contains the conserved Glu and Arg residues (Glu69, Arg70) responsible for gating the CsoS1D pore as well as the universally conserved edge Lys residues in the N- and C-terminal domains (Lys108, Lys212) for interacting with other hexamers to incorporate into the shell (Cai et al. in press). A second ~200 amino acid BMC-domain protein is found only in low-light adapted strains of Prochlorococcus and some marine Synechococcus species.

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