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Crystal structures of ?-carboxysome shell protein CcmP: ligand binding correlates with the closed or open central pore.

ABSTRACT: Cyanobacterial CO2 fixation is promoted by encapsulating and co-localizing the CO2-fixing enzymes within a protein shell, the carboxysome. A key feature of the carboxysome is its ability to control selectively the flux of metabolites in and out of the shell. The ?-carboxysome shell protein CcmP has been shown to form a double layer of pseudohexamers with a relatively large central pore (~13 Å diameter), which may allow passage of larger metabolites such as the substrate for CO2 fixation, ribulose 1,5-bisphosphate, through the shell. Here we describe two crystal structures, at 1.45 Å and 1.65 Å resolution, of CcmP from Synechococcus elongatus PCC7942 (SeCcmP). The central pore of CcmP is open or closed at its ends, depending on the conformation of two conserved residues, Glu69 and Arg70. The presence of glycerol resulted in a pore that is open at one end and closed at the opposite end. When glycerol was omitted, both ends of the barrel became closed. A binding pocket at the interior of the barrel featured residual density with distinct differences in size and shape depending on the conformation, open or closed, of the central pore of SeCcmP, suggestive of a metabolite-driven mechanism for the gating of the pore.

SUBMITTER: Larsson AM

PROVIDER: S-EPMC5853799 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature

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Crystal structures of β-carboxysome shell protein CcmP: ligand binding correlates with the closed or open central pore.

Larsson Anna M AM Hasse Dirk D Valegård Karin K Andersson Inger I

Journal of experimental botany 20170601 14

Cyanobacterial CO2 fixation is promoted by encapsulating and co-localizing the CO2-fixing enzymes within a protein shell, the carboxysome. A key feature of the carboxysome is its ability to control selectively the flux of metabolites in and out of the shell. The β-carboxysome shell protein CcmP has been shown to form a double layer of pseudohexamers with a relatively large central pore (~13 Å diameter), which may allow passage of larger metabolites such as the substrate for CO2 fixation, ribulos ...[more]

PMID: 28369612

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Project description:A series of oligothiophene bis(dioxolene) complexes, SQ-Thn-SQ (SQ = S = ½TpCum,MeZnII(3-tert-butyl-orthosemiquinonate); TpCum,Me = tris(5-cumenyl-3-methylpyrazolyl)borate anion) have been synthesized, structurally characterized, and studied as a function of the number of thiophene bridging units, n (n = 0-3) using a combination of variable-temperature (VT) electronic absorption and EPR spectroscopies, and VT magnetic susceptibility measurements. The thiophene bridge bond lengths determined by X-ray crystallography display dramatic differences across the SQ-Thn-SQ series. Bridge bond deviation values (Σ|Δi|) display a progressive change in the nature of the bridge fragment bonding as the number of thiophene groups increases, with quinoidal bridge character for n = 1 (SQ-Th-SQ) and biradical character with "aromatic" bridge bond lengths for n = 3 (SQ-Th3-SQ). Remarkably, for n = 2 (SQ-Th2-SQ) the nature of the bridge fragment is intermediate between quinoid and biradical aromatic, which we describe as having open-shell character as opposed to biradicaloid since the open-shell biradical configuration does not have the correct symmetry to mix with the quinoidal ground-state configuration. This bridge bonding character is reflected in the energies of the lowest lying open-shell states for these three molecules. The SQ-Th-SQ molecule is diamagnetic at all temperatures studied, and we provide evidence for SQ-SQ antiferromagnetic exchange coupling and population of triplet states in SQ-Th2-SQ and SQ-Th3-SQ, with JSQ-SQ(ave) = -279 cm-1 (VT EPR/electronic absorption/magnetic susceptibility) and JSQ-SQ = -117 cm-1 (VT EPR/electronic absorption/magnetic susceptibility), respectively. The results have been interpreted in the context of state configurational mixing within a simplified 4-electron, 3-orbital model that explicitly contains contributions of a bridge fragment. Variable-temperature spectroscopic- and magnetic susceptibility data are consistent with two low-lying open-shell states for SQ-Th3-SQ, but three low-lying states (one closed-shell and two open-shell) for SQ-Th2-SQ. This model provides a simple symmetry-based framework to understand the continuum of electronic and geometric structures of this class of molecules as a function of the number of thiophene units in the bridge.

Dataset Information

Crystal structures of ?-carboxysome shell protein CcmP: ligand binding correlates with the closed or open central pore.

Publications

Crystal structures of β-carboxysome shell protein CcmP: ligand binding correlates with the closed or open central pore.

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