Synthetic xylan-binding modules for mapping of pulp fibres and wood sections

The complex carbohydrate composition of natural and refined plant material is not known in detail but a matter that is of both basic and applied importance. Qualitative assessment of complex samples like plant and wood tissues requires the availability of a range of specific probes. Monoclonal antibodies and naturally existing carbohydrate binding modules (CBMs) have been used in the past to assess the presence of certain carbohydrates in plant tissues. However, the number of natural CBMs is limited and development of carbohydrate-specific antibodies is not always straightforward. We envisage the use of sets of very similar proteins specific for defined targets, like those developed by molecular evolution of a single CBM scaffold, as a suitable strategy to assess carbohydrate composition. An advantage of using synthetic CBMs lies in the possibility to study fine details of carbohydrate composition within non-uniform substrates like plant cell walls as made possible through minor differences in CBM specificity of the variety of binders that can be developed by genetic engineering.

A panel of synthetic xylan-binding CBMs, previously selected from a molecular library based on the scaffold of CBM4-2 from xylanase Xyn10A of Rhodothermus marinus, was used in this study. The wild type CBM4-2 and evolved modules both showed binding to wood sections. However, differences were observed in the staining patterns suggesting that these modules have different xylan-binding properties. Also the staining stability varied between the CBMs, the most stable staining being obtained with one (X-2) of the synthetic modules. Treatment of wood materials resulted in altered signal intensities, thereby also demonstrating the potential application of engineered CBMs as analytical tools for quality assessment of diverse plant material processes.

In this study we have demonstrated the usefulness of synthetic xylan-binding modules as specific probes in analysis of hemicelluloses (xylan) in wood and fibre materials.