Deep inside the cells of Chloroflexus aurantiacus, a bacterium that harnesses sunlight for energy, a molecular gatekeeper called Chy400_4166 has been quietly performing a job scientists are only now beginning to understand. Researchers at Tokyo University of Science have revealed how this protein acts as a selective bouncer, allowing rare sugar molecules called β-1,2-glucans to cross bacterial cell membranes—a discovery that illuminates a hidden survival strategy bacteria use to thrive in their environments.
The significance of this work extends far beyond the laboratory. β-1,2-glucans are not simple fuel; they are structurally complex sugar polymers that play surprisingly sophisticated roles across the living world. The pathogenic bacterium Brucella abortus, for instance, deploys these molecules as a shield, producing them to evade destruction inside immune cells and enable infection. Plant pathogens like Xanthomonas species rely on β-1,2-glucans to establish infections in crops and model plants such as Arabidopsis thaliana and Nicotiana benthamiana. Yet despite decades of research into how bacteria produce and degrade these molecules, scientists had overlooked a fundamental question: how do bacteria actually import them across their cell membranes?
Associate Professor Masahiro Nakajima and Professor Hidetaka Torigoe from Tokyo University of Science, alongside Associate Professor Hiroyuki Nakai from Niigata University, set out to answer that question. Their team, publishing in The FEBS Journal, identified and mapped the three-dimensional structure of Chy400_4166 at atomic resolution—achieving clarity down to 1.27 to 1.95 angstroms using X-ray crystallography. This protein is part of an ABC transporter, a cellular machine that uses energy to pump specific molecules into cells.
What makes Chy400_4166 remarkable is not just what it does, but how it does it differently from any known β-1,2-glucan importer. Using gel shift assays and isothermal titration calorimetry, the researchers discovered that the protein binds to a middle segment of longer glucan chains—a position perfectly suited for handling the cyclic forms of these molecules. This is a fundamentally different strategy than the only other characterized β-1,2-glucan binding protein, from the bacterium Listeria innocua, which grips the short ends of sugar chains. The researchers found that 10 consecutive glucose units form the core-binding interface across all substrates tested, with a single unit, designated G, tightly anchored by highly conserved amino acid residues. Perhaps most intriguingly, Chy400_4166 displays structural flexibility, allowing it to shift its conformation to accommodate cyclic glucans of varying sizes.
The protein showed strong binding affinity for both linear and cyclic β-1,2-glucans while rejecting a structurally different barley-derived glucan, confirming its exquisite selectivity. "These findings are intriguing in that they imply a remarkable diversity among β-1,2-glucan-associated binding proteins," Dr. Nakajima noted. This diversity matters profoundly. Previous studies of β-1,2-glucan transport systems suggested they varied considerably from one another, implying that bacteria have evolved multiple distinct strategies to import these crucial molecules. Now, with Chy400_4166 characterized, the full scope of that biological diversity is beginning to emerge, offering new pathways for understanding how pathogens infiltrate their hosts and how bacteria survive in changing environments.