Functional Characterization of a β-1,3-Glucanosyltransferase CmGel4 in Cordyceps militaris Using a Precise CRISPR-Cas9 Genome-Editing System

Cordyceps militaris polysaccharides, especially β-glucans, have presented significant antitumor, hypoglycemic, and immunomodulatory activities. However, the enzymes involved in the branching formation of C. militaris β-glucans remain to be elucidated. In the present study, a 1.69-kb β-1,3-glucanosyltransferase CmGel4 gene putatively involved in β-glucan branching was cloned from C. militaris mycelia and bioinformatically analyzed. The encoded 54.12 kDa CmGel4p consisted of 515 amino acid residues and contained a typical GH72⁺ structural characteristic of a signal peptide (1-19aa), a GH72 conserved domain (20-334aa), a GPI-anchor site (485aa), and a CBM43/X8 domain (382-458aa). Using the established CRISPR-Cas9 genome-editing system, the full length of 1.69-kb CmGel4 was precisely inserted at a genomic safe-harbor site CmSh1, and the GH72 conserved domain of CmGel4 was successfully deleted in C. militaris genome for the first time. By comparing the mycelial growth and fermentation performance of WT, control, and CmGel4-overexpressed/knockout mutants, β-1,3-glucanosyltransferase gene CmGel4 was shown to play key roles in cell growth and branching of exo-polysaccharides of C. militaris, accompanied by the transcriptional changes of genes such as CmGel4, CmUgp, and CmPgm. These findings provided the proof of β-1,3-glucanosyltransferases vital for formatting cell walls and maintaining cellular integrity, and a fine regulation strategy for precisely remodeling the β-1,3-glucan with high-branched structures in edible fungi.