This illustration shows how hexagonal bacterial proteins (shown as ribbon-like structures at right and upper right) self-assemble into a honeycomb-like tiled pattern (center and background). This tiling activity, seen with an atomic-resolution microscope (upper left), represents the early formation of polyhedral, soccer-ball-like structures known as bacterial microcompartments or BCMs that serve as tiny factories for a range of specialized activities. Credit: Berkeley Lab
Scientists have for the first time viewed how bacterial proteins self-assemble into thin sheets and begin to form the walls of the outer shell for nano-sized polyhedral compartments that function as specialized factories. The study provides new clues for 3D structures as “nanoreactors” to selectively suck in toxins or churn out desired products. It may aid scientists tapping natural origami by designing novel compartments or using them as scaffolding for new types of nanoscale architectures, such as drug-delivery systems.
Researchers combined X-ray studies of the 3-D structure of a protein that resembles a hexagon with imaging by an atomic-force microscope to reveal how the hexagons arrange in a honeycomb pattern in the microcompartment’s walls. Patterns produced when X-rays struck the protein crystals provided key details about the protein’s shape, at the scale of individual atoms. “That gave us some exact dimensions,” Sutter said, which helped to interpret the microscope images. “It also showed us that hexagons had distinct sidedness: One side is concave, the other side is convex.”
Liverpool’s atomic-force microscope, BioAFM, showed hexagon-shaped protein pieces naturally join to form ever-larger protein sheets in a liquid solution. The hexagons only assembled with each other if they had the same orientation -convex with convex or concave with concave. These hexagon-shaped pieces of the protein sheet can also dislodge and move from one protein sheet to another. Such dynamics may allow fully formed compartments to repair individual sides.
The protein sheets studied were not viewed inside living bacteria, though the conditions of the microscope experiment were designed to mimic those of the natural bacterial environment. This study suggests the shell facets are composed of a single protein layer. The compartments selectively allow some chemical exchanges between their contents and outside environment, and a thicker shell could complicate these exchanges.
The exact mechanism for this chemical exchange is not yet well-understood. This and other mysteries of the microcompartments can hopefully be resolved with follow-up studies. Fully-formed 3-D microcompartments have a soccer-ball-like geometry that incorporates pentagon-shaped protein structures known as pentamers, for example. It’s possible that simply adding these pentamers to the protein sheets from the latest experiment could stimulate the growth of a complete 3-D structure, but Kerfeld added, “I wouldn’t be surprised if there’s more to the story.”
Once more is learned about the microcompartments, they could be used to concentrate the production of beneficial enzymes, organize them to produce an ordered sequence of chemical reactions, or to remove particular toxins from the surrounding environment, she said. http://www.eurekalert.org/pub_releases/2015-12/dbnl-nmt122115.php
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