Chaperones fold Rubisco
Chaperones ensure good order and form
That is why the Martinsried-based researchers want to modify Rubisco in such a way that it can bind only carbon dioxide. To do this, they must first establish how the protein is actually formed. Rubisco is one of the largest of all proteins and consists of eight large and eight small subunits. “With so many subunits, there is a significant risk that the wrong parts of the protein will aggregate and clump together,” explains Manajit Hayer-Hartl. In order for the protein to function correctly, the amino acid chains must be correctly folded and the subunits assembled so that they form a cylinder. This complex folding process is managed by special proteins known as chaperones.
According to the researchers, three proteins are required to recreate a functional Rubisco complex: in addition to the previously identified chaperonins GroEL and GroES, a recently discovered helper protein (RbcX). RbcX ensures that two large subunits of Rubisco can assemble next to each other. Four of these dimers then form the cylinder, and four small subunits position themselves at the top and bottom areas of the cylinder. “We now understand why, for example, bacteria are not able to produce functional Rubisco. If we insert only the DNA for the protein into the bacterial genome – without the corresponding helper protein – functional Rubisco cannot be formed,” says Ulrich Hartl.
Having achieved this breakthrough, the scientists can now get to work on producing Rubisco artificially in the laboratory. To this end, they want to introduce the DNA for Rubisco, the two chaperonins and the helper protein into bacteria. The rapidly reproducing microorganisms will then produce the Rubisco protein in sufficient quantities. The researchers are seeking to find a more efficient variant of Rubisco with the help of such bacteria. “If we introduce the Rubisco DNA into a bacterial strain that can survive only with functional Rubisco, we can test all possible mutations in the Rubisco gene and immediately establish how well the individual variants work,” explains Manajit Hayer-Hartl.
Can humans succeed where nature failed?
With the help of this process, multiple mutations can also be generated and studied in different positions in the Rubisco gene. This is an important advantage, as it may not be possible to further optimize the protein by replacing a single amino acid. This would explain why nature itself did not adapt Rubisco to the increasing oxygen content in the air over the course of evolution. Some scientists believe that nature has already found the optimum structure for Rubisco, and that Rubisco cannot be improved. The scientists in Martinsried disagree. They are convinced “that the Rubisco molecule of plants is definitely not the best possible variant. Some red algae have a more efficient form. This suggests that the plant enzyme can be improved, too.”
However, finding mutations that would render Rubisco more specific to carbon dioxide is not the only challenge the scientists have to face. The new results show that nothing will work without the matching molecular chaperones. Unlike Rubisco itself, RbcX works extremely selectively and also assists in the folding of the natural plant Rubisco. For this reason, it has not been possible, for example, to transfer the red-algal Rubisco to plants – it simply does not fold correctly in this case. It is thus possible that an optimized Rubisco variant will also require its own specific assembly chaperone.