Glomalin-Related Soil Protein Contains Thioredoxin, Non-Mycorrhizal-Related Heat-Stable Proteins, Lipids and Humic Materials.

Glomalin-related soil protein (GRSP) is a stable and persistent material produced in copious quantities by mycorrhizal fungi and may be an important pool of organic N in soil. Glomalin-related soil protein, however, is only operationally defined and has been only poorly characterized at best.  Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and pyrolysis field-ionization mass-spectrometry (Py-FIMS) revealed that GRSP contains a consortium of proteins along with many impurities. Employing proteomic techniques, we found that glomalin itself may be a thioredoxin-containing chaperone; however, no homologies with mycorrhizal proteins or DNA were detected.  Proteomics techniques further revealed that this fraction contains large amounts of soil-related heat-stable proteins and proteins of non-mycorrhizal origin. Results of this research show that the current extraction procedure that defines GRSP produces a mixture of compounds and thereby overestimates glomalin stocks when quantified using the Bradford assay.  The chemical nature of glomalin has yet to be conclusively determined; it is unlikely that the chemical structure of glomalin can be elucidated from the mixture extracted as GRSP. Further investigations into the structure of glomalin should instead focus on isolating and purifying protein from gnotobiotic mycorrhizal cultures.