Cyanobacteria fix carbon within carboxysomes. Here, RubisCO and carbonic anhydrase are coencapsulated within a semipermeable protein shell built from paralogs of the CcmK proteins. Crystal packing patterns suggest that the shell facets may be built as a single layer of CcmK molecules tiled hexagonally in a continuous sheet. We used fluorescence resonance energy transfer (FRET) to measure interactions mediated by CcmK paralogs from Thermosynechococcus elongatus. CcmK2-an abundant, universally present paralog-shows uniquely strong self-interactions. The CcmK2 structure reveals a back-to-back dodecameric organization, with interactions mediated by a helix comprised of residues 95-101. Modeling indicates that this dodecameric interaction could seamlessly fuse two sheets into a double-layered shell. This model predicts several aspects of CcmK2 interactions, including the attenuation of FRET by Glu95Ala variants at the dodecameric interface. This model also accurately predicts the observed shell thickness, implying that the β-carboxysome shell is most likely organized as a double layer.
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