Variety of size and form of GRM2 bacterial microcompartment particles

Abstract

Bacterial microcompartments (BMCs) are bacterial organelles involved in enzymatic processes, such as carbon fixation, choline, ethanolamine and propanediol degradation, and others. Formed of a semi‐permeable protein shell and an enzymatic core, they can enhance enzyme performance and protect the cell from harmful intermediates. With the ability to encapsulate non‐native enzymes, BMCs show high potential for applied use. For this goal, a detailed look into shell form variability is significant to predict shell adaptability. Here we present four novel 3D cryo‐EM maps of recombinant Klebsiella pneumoniae GRM2 BMC shell particles with the resolution in range of 9 to 22 Å and nine novel 2D classes corresponding to discrete BMC shell forms. These structures reveal icosahedral, elongated, oblate, multi‐layered and polyhedral traits of BMCs, indicating considerable variation in size and form as well as adaptability during shell formation processes.

Heterologous Assembly of Pleomorphic Bacterial Microcompartment Shell Architectures Spanning the Nano‐ to Microscale

“…Interestingly, we also observed examples of nested shells, with one or more smaller shells encapsulated within a larger polyhedron (Figure 2C, white arrow), similar to those reported previously. [ 5 ] These results demonstrate that the CAP method can successfully isolate shells of drastically different sizes and shapes, including the unique capped nanotubes and ovoid morphologies that had not been characterized previously.…”

“…For the β ‐carboxysome, there is one hexamer belt between icosahedral hemispheres, [ 4 ] whereas GRM2 shells have been observed with 1 to 4 belts of BMC‐H. [ 5,11 ] Despite their substantially shorter lengths compared to the GRM3C nanotubes, the end caps sealing these short prolates can theoretically be used to close a tube of any length. This geometric model has also been used as a framework for interpreting the structures of a number of short prolate viral capsids, [ 38 ] as well as elongated (≈50–100 nm) alfalfa mosaic virus (AMV) nanotubes.…”

“…Although the geometry of the cylindrical body of a GRM3C nanotube can be explained by the rolling-up of a 2D hexagonal array of shell proteins like open-ended BMC-H or BMC-T nanotubes, the observation of end-capped nanotubes in our electron microscopy images (Figure 3C), and the involvement of BMC-P strep (selected for by our affinity-based purification method), points to important differences in their structures and underlying assembly principles. Reports of smaller, ≈20-40 nm long, prolate BMC shells derived from 𝛽-carboxysome [4] and GRM2 [5,11] model shells provide some insight into how the end caps can be formed by analogy to those described for carbon nanotubes. [37] For example, BMC-H and BMC-P can close a cylindrical tube made of hexagonal shell proteins by splitting an icosahedron into two equal halves, with the circumference of the icosahedron dictating the nanotube diameter, and a variable number of hexamer belts inserted between the caps, dictating the nanotube length.…”

“…[37] For example, BMC-H and BMC-P can close a cylindrical tube made of hexagonal shell proteins by splitting an icosahedron into two equal halves, with the circumference of the icosahedron dictating the nanotube diameter, and a variable number of hexamer belts inserted between the caps, dictating the nanotube length. For the 𝛽-carboxysome, there is one hexamer belt between icosahedral hemispheres, [4] whereas GRM2 shells have been observed with 1 to 4 belts of BMC-H. [5,11] Despite their substantially shorter lengths compared to the GRM3C nanotubes, the end caps sealing these short prolates can theoretically be used to close a tube of any length. This geometric model has also been used as a framework for interpreting the structures of a number of short prolate viral capsids, [38] as well as elongated (≈50-100 nm) alfalfa mosaic virus (AMV) nanotubes.…”

“…Bacterial microcompartments (BMCs) are self-assembling subcellular organelles consisting of a core of catabolic (metabolosomes) or anabolic (carboxysomes) enzymes surrounded by a shell composed of hexameric (BMC-H), trimeric (BMC-T), and pentameric (BMC-P) proteins. [1] When expressed in the absence of their native enzymatic cargo, BMC shell proteins and their engineered derivatives self-assemble into an array of higherorder structures, including 2D hexagonal sheets, [2,3] small prolates, [4,5] open-ended nanotubes, [2,[6][7][8] dodecahedra, [9] icosahedra with diameters up to 40 nm, [4,[10][11][12][13][14] and irregular polyhedra. [8,11,15] The building blocks of these genetically tractable BMC-based biomaterials are structurally redundant, forming either hexagons (BMC-H and BMC-T) [16,17] or pentagons (BMC-P).…”