FCS Single-Molecule Analyzer Aids IGDB Team in Revealing Xylan Polymerization Dynamics

Xylan is the second most abundant polysaccharide in plant cell walls after cellulose, and plays critical roles in maintaining cell wall integrity, mechanical strength, and biomass recalcitrance. Its biosynthesis relies on a multi-enzyme complex known as the xylan synthase complex (XSC). However, the core components and biochemical mechanisms of XSC remain largely unknown. Recently, a research team led by Baocai Zhang from the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences (IGDB, CAS) published a study in The Plant Cell entitled XYLAN O-ACETYLTRANSFERASE 6 promotes xylan synthesis by forming a complex with IRX10 and governs wall formation in rice.

This study identified XYLAN O-ACETYLTRANSFERASE 6 (XOAT6) and IRX10 (a xylan synthase) as core components of XSCs, forming the functional core module of the complex. The results demonstrated that XOAT6 not only acetylates the xylan backbone but also directly enhances the polymerase activity of IRX10. Together, they coordinately and efficiently synthesize acetylated xylan. For the first time at the molecular level, this work reveals the mechanisms underlying xylan chain elongation and modification, providing a theoretical basis for cell wall engineering, breeding high-yield and high-quality crops, and developing efficient biomass energy.

The study hypothesized that XOAT6 acts not only as an acetyltransferase but also directly promotes backbone polymerization (elongation) of xylan by forming a complex with IRX10. To test this, Fluorescence Correlation Spectroscopy (FCS) was used to monitor real-time kinetic changes during polymerization, aiming to explore whether and how XOAT6 influences IRX10 polymerase activity under near-physiological solution conditions.

As a glycosyltransferase, IRX10 functions by adding new xylosyl units (donor) to the end of an existing xylan chain (acceptor). In this study, xylobiose (X2) was used as the initial acceptor, and UDP-xylose as the donor substrate.

X2 was labeled with the fluorescent dye Alexa Fluor 488 (Figure 4B). The hydrodynamic radius (RH) of fluorescently labeled molecules was measured via autocorrelation function for dynamic tracking, reflecting xylan chain elongation in real time.

• Control groups: Labeled X2 + donor only; or labeled X2 + enzyme (without donor).
• Experimental groups: IRX10 + donor + labeled X2 IRX10 + XOAT6 + donor + labeled X2

• XOAT6 strongly promotes polymerization: When both IRX10 and XOAT6 were present, the change in RH was far more significant than with IRX10 alone, indicating that XOAT6 enhances the polymerase activity of IRX10 (Figure 4C).
• RH changes arise from polymerization: Control experiments showed no RH shift when reactions contained only donor substrate (UDP-Xyl) and AF488-labeled X2, or only enzyme and labeled X2 without donor (Figure 4C). This confirms that RH changes result specifically from polymerization of UDP-Xyl onto AF488-labeled X2.
• Real-time monitoring of xylan polymerization: Continuous measurements further revealed that the rate of RH increase approximately doubled when IRX10 and XOAT6 acted together, directly proving their synergistic promotion of xylan chain elongation and markedly improved polymerization efficiency at the molecular level (Figure 4D).

Figure 4 Validation of molecular interactions among XOAT6, IRX10, and their substrates. (A) MST assay: Binding affinity (Kd) of XOAT6 and its mutants to labeled IRX10. (B–D) FCS assays: AF488-labeled xylobiose was used to monitor changes in RH in real time, reflecting xylan chain elongation.

In this study, Fluorescence Correlation Spectroscopy (FCS) was used to observe dynamic changes during xylan polymerization in real time at the single-molecule level, showing clear strengths especially in characterizing how the IRX10–XOAT6 complex promotes xylan chain elongation:

• Single-molecule real-time dynamic monitoring: FCS detects diffusion behavior of individual fluorescently labeled molecules at ultra-low concentrations (pM–nM), allowing real-time tracking of hydrodynamic radius changes during polymerization. In contrast, traditional methods such as mass spectrometry and electrophoresis only analyze end-point products and cannot provide time-resolved dynamic data.
• In-solution, native detection: Assays are performed in homogeneous solution under near-physiological conditions, without sample immobilization or disruption of native protein conformations. Electron microscopy and NMR require fixation or drying, which may alter the native state of protein complexes.

This study reveals, for the first time, the central role of the IRX10–XOAT6 complex in rice xylan biosynthesis. It systematically elucidates how this complex affects plant growth, development, and biomass properties through coordinated catalysis and structural regulation across molecular mechanism, cell wall architecture, and biomechanics. These findings provide an important theoretical foundation and technical support for crop genetic improvement, efficient biomass utilization, and plant cell wall biology.

FCS was applied to plant cell wall polysaccharide biosynthesis for the first time, overcoming limitations of conventional methods. It enables real-time, single-molecule dynamic visualization and quantification of biomolecular interactions and enzymatic reactions in solution under near-physiological conditions, establishing a new paradigm for studying the synthesis mechanisms of such macromolecules. As a powerful technique for real-time monitoring of biomolecular dynamics at the single-molecule level in solution, FCS holds broad promise, especially for dynamic biological processes, drug development, and nanotechnology, and is expected to drive life science research from static structural analysis toward precise regulation of dynamic processes.

Original link: https://doi.org/10.1093/plcell/koae322

Fluorescence Correlation Spectroscopy (FCS) quantitatively measures molecular/nanoparticle properties—including molar concentration, fluorescence brightness/aggregation state, diffusion coefficient/hydrodynamic radius, and interaction affinity (KD)—at single-molecule resolution in microliter-scale solution samples or single living cells. It is an in-situ, homogeneous, high-content tool compatible with physiological samples (cell lysates, blood, etc.).

FCS has been widely used in cell signaling, liquid–liquid phase separation, biomolecular denaturation and aggregation, structure–function mechanisms, nanomedicine development, exosome analysis, fluorescent probe engineering, antibody and drug screening, and microfluidics. More than 15,000 publications in PubMed feature FCS and its derivative technologies.