Identification of a fungal protein of Syncephalastrum racemosum as aspartic proteinase

Pathogenic nature of Syncephalastrum in Atta sexdens rubropilosa fungus gardens

BACKGROUND

Leaf-cutter ants are considered to be a major herbivore and agricultural pest in the Neotropics. They are often controlled by environmentally persistent insecticides. Biological control using pathogenic fungi is regarded as an alternative for the management of these insects. Here, we assess whether the filamentous fungus Syncephalastrum sp. is a pathogenic microorganism responsible for a characteristic disease in fungus gardens. We also characterise the damage caused by this fungus by evaluating physiological and behavioural responses of Atta sexdens rubropilosa subcolonies infected with Syncephalastrum sp.

RESULTS

Syncephalastrum sp. fulfils Koch's postulates characterising it as a pathogenic microorganism. Ant workers recognise the infection and remove contaminated fragments from the fungus garden. Syncephalastrum sp. infection causes an interruption of foraging activity, an increase in ant mortality, subcolony deterioration and an increase in the amount of waste generated, all resulting in subcolony death. Syncephalastrum sp. also inhibits the ant fungal cultivar in vitro. The pathogenic effect of Syncephalastrum sp. does not depend on host morbidity or stress (e.g. worker mortality caused by an entomopathogenic fungus).

CONCLUSION

Syncephalastrum sp. treatment resulted in progressive damage in subcolonies. The interactions among Syncephalastrum sp., fungus garden and ants offer new opportunities in integrated pest management of leaf-cutter ants. © 2016 Society of Chemical Industry.

Purification and characterization of a new Rhizopuspepsin from Rhizopus oryzae NBRC 4749

A secretory aspartic protease (also termed as rhizopuspepsin) was purified from Rhizopus oryzae NBRC 4749 by ion exchange chromatography with a yield of 45%. The enzyme was a nonglycoprotein with a molecular mass of 37 kDa as determined by SDS-PAGE analysis. N-terminal sequence and LC-MS/MS analyses revealed that this rhizopuspepsin corresponded to the hypothetical protein RO3G_12822.1 in the R. oryzae genome database. Comparison of genomic and cDNA genes demonstrated that the rhizopuspepsin contained two introns, whereas only one intron was reported in other rhizopuspepsin genes. Phylogenetic analysis also indicated that this rhizopuspepsin was distinct from other rhizopuspepsins. The temperature and pH optima for the purified rhizopuspepsin were 50 degrees C and pH 3.0, respectively, and a half-life of about 3.5 h was observed at 40 degrees C. The enzyme preferentially cleaved the peptides with hydrophobic and basic amino acids in the P1 site but had no activity for the Glu, Pro, Trp, and aliphatic amino acids containing the beta-branch side chain.

Single-column purification of syncephapepsin--an aspartic proteinase from Syncephalastrum racemosum

Syncephapepsin is a fungal aspartic proteinase from Syncephalastrum racemosum. By using the property of syncephapepsin after having increased activity at higher temperature, two rapid purification protocols were developed. In the former case, a crude extract was initially diluted fivefold with an activity assay buffer and heated at 50 degrees C overnight. In this situation, syncephapepsin would digest most of the proteins that the crude extract contained. Subsequently, syncephapepsin of the crude extract was precipitated from 50 to 70% of ammonium sulfate and the preparation was then directly applied to the Superdex 200 HR FPLC column. In this manner, syncephapepsin was rapidly purified to apparent homogeneity within 24 h. In this report, an alternative method of purification is also provided. Compared with the procedure mentioned above, the heating step was proceeded after FPLC chromatography through which the same result was obtained. Using cytochrome c and RNase A as substrates, the cleavage sites of both substrates were identified by HPLC peptide mapping. The results showed that syncephapepsin had a broad specificity. Residues recognized to be cleaved were primarily those of trypsin and chymotrypsin and Lys was the most susceptible.