Department of Plant Biology

Genetics Program

Walton Lab

Natural Products and Enzymes from Fungi

Two salient features of the Kingdom Mycota (fungi) are the synthesis of biologically active natural products and the secretion of degradative enzymes. Both have a strong impact on natural ecosystems and on human activities. Many of our most important pharmaceutical agents are derived from fungi, such as penicillin, cyclosporin, and statins. Secreted enzymes are the means by which fungi have become the dominant recyclers of carbon in all terrestial ecosystems. Our lab studies important ecological and biotechnological aspects of both fungal natural products and fungal degradative enzymes.

Among the ecologically significant natural products made by fungi are host-selective toxins, which account for the specificity and high virulence of certain plant pathogenic fungi. Host-selective toxins have been implicated in at least two major crop epidemics in the U.S. We have characterized the complex genetic locus responsible for production of the cyclic tetrapeptide HC-toxin by the maize pathogen Cochliobolus carbonum. The central enzyme is a 570-kDa nonribosomal peptide synthetase encoded by a 16-kb open reading frame. Furthermore, we have shown that the biochemical basis of resistance in maize to C. carbonum is an enzyme that detoxifies HC-toxin. The site of action of HC-toxin is histone deacetylase, an enzyme that regulates gene expression via alteration of chromatin structure.

Another important group of fungal natural products are the bicyclic peptides (amatoxins and phallotoxins) made by mushrooms in the genus Amanita, which account for >90% of fatal mushroom poisonings. In contrast to all other known fungal cyclic peptides, including HC-toxin, the Amanita toxins are synthesized on ribosomes instead of by nonribosomal peptide synthetases. We are currently actively working on the biosynthetic pathway of the Amanita toxins.

Figure 1. Conocybe albipes growing in a lawn at MSU. This mushroom makes the bicyclic peptide phalloidin (inset structure) and was used as the source to purify and identify prolyl oligopeptidase, which processes the phalloidin precursor protein to release the linear heptapeptide of mature phalloidin (see Luo et al., 2009).

Building on our years of work on the role in plant pathogenesis of secreted cell-wall-degrading enzymes, such as pectinase, xylanase, and cellulase, we are now studying their biotechnological applications for biomass conversion. The focus of this work, under the auspices of the Great Lakes Bioenergy Research Center, is to identify the key enzymes for deconstruction of plant cell walls to fermentable sugars with the long-term goal of producing more efficient enzyme cocktails. Our principal strategy is to construct optimized synthetic mixtures using individual pure enzymes, statistical experimental design, and robotic liquid handling. Mixtures containing more than 18 components have been developed and shown to equal or exceed commercial “cellulase” preparations.

Figure 2.Optimized mixtures of eleven enzymes for release of Glc or Xyl from pretreated corn stover (Banerjee et al., 2010).