1.S.3.  The Bacterial Microcompartment Shell/Pore-forming Protein-3 (BMC-SP3) Family  

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 forms 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. Larsson et al. 2017 described 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 features 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 (Larsson et al. 2017).