This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The assembly pathway behind refractile-body proteins may have finally been resolved, researchers from Science Tokyo report. Using genetic engineering and several techniques for analyzing protein structure, they investigated how four proteins work together to produce the correct architecture of these complex, pH-responsive protein machines, from the nanoscale to the microscale.
The findings could help establish new frameworks for engineering dynamic protein systems as biotechnological tools. Some proteins can undergo drastic structural transformations in response to subtle changes in their surroundings. Refractile bodies (R-bodies), protein assemblies found in certain bacteria that are endosymbionts of Paramecium, are a prime example.
Under relatively neutral pH conditions, R-bodies remain tightly coiled, like a rolled-up ribbon. When their surroundings become acidic, however, they rapidly extend into a spiral structure about 50 times longer than their rolled form. This extension happens within a second and can generate enough mechanical force to disrupt a cell membrane.
Although R-bodies were discovered more than 70 years ago, scientists still have a limited understanding of how these protein machines are built. Type 51 R-bodies, one of the more studied varieties, are made from four proteins called RebA, RebB, RebC and RebD, coded by a four-gene operon comprising rebA, rebB, rebC and rebD. RebA and RebB are thought to be the main structural components of R-bodies, but the roles of RebC and RebD remain unclear.
How do these four proteins work together to create the complex, ordered nanoscale architecture seen in R-bodies? A research team led by graduate student Koki Date, Assistant Professor Kosuke Kikuchi and Professor Takafumi Ueno from the School of Life Science and Technology, Institute of Science Tokyo (Science Tokyo), Japan, set out to answer this question. Their study, published online in the journal Biomacromolecules on Sept. 18, 2026, combines genetic engineering with advanced microscopy, fluorescence assays, X-ray scattering and spectroscopy to shed light on the function of each protein involved in assembling a type 51 R-body.
The researchers systematically examined what happens when each of the four reb genes is deleted from plasmids, both one at a time and in various combinations, using single-, double- and triple-gene knockout mutants expressed in Escherichia coli. They then examined these mutants in detail, comparing their shapes, internal structures and protein composition with those of a fully functional wild-type R-body. The results showed that RebD and RebC are critical to the formation of R-body architecture.
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