ABSTRACTChitin is the second most abundant polysaccharide, present, e.g., in insect and arthropod exoskeletons and fungal cell walls. In some species or under specific conditions, chitin appears to be enzymatically de-N-acetylated to chitosan—e.g., when pathogenic fungi invade their host tissues. Here, the deacetylation of chitin is assumed to represent a pathogenicity mechanism protecting the fungus from the host's chitin-driven immune response. While highly specific chitin binding lectins are well known and easily available, this is not the case for chitosan-specific probes. This is partly due to the poor antigenicity of chitosan so that producing high-affinity, specific antibodies is difficult. Also, lectins with specificity to chitosan have been described but are not commercially available, and our attempts to reproduce the findings were not successful. We have, therefore, generated a fusion protein between a chitosanase inactivated by site-directed mutagenesis, the green fluorescent protein (GFP), and StrepII, as well as His6tags for purification and detection. The recombinant chitosan affinity protein (CAP) expressed inEscherichia coliwas shown to specifically bind to chitosan, but not to chitin, and the affinity increased with decreasing degree of acetylation.In vitro, CAP detection was possible either based on GFP fluorescence or using Strep-Tactin conjugates or anti-His5antibodies. CAP fluorescence microscopy revealed binding to the chitosan exposing endophytic infection structures of the wheat stem rust fungus, but not the chitin exposing ectophytic infection structures, verifying its suitability forin situchitosan staining.
Diversifying selection in the wheat stem rust fungus acts predominantly on pathogen-associated gene families and reveals candidate effectors
