Fungal GAPDHs: role beyond glycolysis
Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is an enzyme of ~37kDa that catalyzes the sixth step (glyceraldehyde 3-phosphate to D-glycerate 1,3-bisphosphate) of glycolysis. GAPDH is a multifunctional enzyme that exhibit several non glycolytic functions. Non glycolytic activities include DNA repair, receptor mediated cell signaling, the cellular response to oxidative stress and to apoptosis etc. While animal cells contain only one isoform, plants contain several isoforms of GAPDH. In contrast with the well established multifunctional properties of animal GAPDH, little is known on the multifunctional roles of plant cytoplasmic GAPDH. Fungal genomes usually harbours a single gene for GAPDH, with no additional moonlighting effect of this glycolytic GAPDH having been reported. Earlier we reported the association of an extra GAPDH gene with a secondary metabolism related gene cluster in Trichoderma virens and Aspergillus spp. A survey of the fungal genome database revealed that T. virens and T. harzianum genomes have two genes for GAPDH, one being associated with a terpene biosynthesis gene cluster. However, Aspergillus genomes harbour multiple isoforms of GAPDH, some being associated with secondary metabolism related gene clusters. We have established the role of a GAPDH in biosynthesis of volatile sesquiterpenes in T. virens. The two GAPDHs of T. virens are having more than 80% similarity in amino acid sequences, and the active site is also conserved. Molecular modeling also revealed that these two GAPDHs are identical in 3D conformation. Interestingly, we find a conserved indel (1 residue) in all the GAPDH proteins which are associated with the of VIR cluster. The deletion (aspartate) can be expected to be localized on the surface loop preceding active site residues. We have expressed and purified the secondary-metabolism related GAPDH in E. coli with an aim to study the biophysical properties and structural basis of this unique catalytic property.