Abstract
Bacterial microcompartments (BMCs) are proteinaceous self-assembling organelles that are widespread among the prokaryotic kingdom. By segmenting key metabolic enzymes and pathways using a polyhedral shell, BMCs play essential roles in carbon assimilation, pathogenesis, and microbial ecology. The BMC shell is composed of multiple protein homologs that self-assemble to form the defined architecture. There is tremendous interest in engineering BMCs to develop new nanobioreactors and molecular scaffolds. Here, we report the quantitative characterization of the formation and self-assembly dynamics of BMC shell proteins under varying pH and salt conditions using high-speed atomic force microscopy (HS-AFM). We show that 400-mM salt concentration is prone to result in larger single-layered shell patches formed by shell hexamers, and a higher dynamic rate of hexamer self-assembly was observed at neutral pH. We also visualize the variability of shell proteins from hexameric assemblies to fiber-like arrays. This study advances our knowledge about the stability and variability of BMC protein self-assemblies in response to microenvironmental changes, which will inform rational design and construction of synthetic BMC structures with the capacity of remodeling their self-assembly and structural robustness. It also offers a powerful toolbox for quantitatively assessing the self-assembly and formation of BMC-based nanostructures in biotechnology applications.
Highlights
Bacterial microcompartments (BMCs) are proteinaceous organelles, structurally resembling viral capsids, that partition the cytoplasm of bacteria [1]
We used the Bacterial microcompartment hexamer (BMC-H) proteins (Hoch_5815) from a myxobacterium Haliangium ocraceum, which were expressed in Escherichia coli and characterized as hexamers with a six-fold symmetry [12]
The patch size increases with the rise of pH from 3 to 10 (Fig. 2a; Additional file 1: Figure S2, Table S1), suggesting that high pH is more favorable for the self-assembly of Hoch_5815 proteins than low pH conditions
Summary
Bacterial microcompartments (BMCs) are proteinaceous organelles, structurally resembling viral capsids, that partition the cytoplasm of bacteria [1]. They are widespread among bacterial phyla [2] and allow bacteria to compartmentalize key metabolic pathways in the absence of membrane-bound organelles found in eukaryotes [3, 4]. The major components of the shell are BMC-H, which appear as hexamers with convex and concave surfaces and tile the Specific protein-protein interactions ensure the self-assembly of BMC proteins to form highly defined architectures to fulfill their metabolic functionality. It has been observed that BMC-H homologs can form various shapes, including two-dimensional sheets [11, 12], nanotubes [13,14,15,16,17], and filament structures [15, 18,19,20]
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