Interpreting the mechanism of action of polyamines: twenty years of plant transglutaminases

The post-translational modification of proteins by polyamines forming inter- or intra-molecular cross-links has been the main transglutaminase (TGase) reaction studied in plants. Plant transglutaminases have been studied in our laboratory since 1987 in investigations aimed at interpreting some of the molecular mechanisms by which polyamines affect plant growth, differentiation, reaction to stresses, PCD. Transglutaminase activity is ubiquitous: it has been detected in algae and in angiosperms in different organs and sub-cellular compartments: chloroplasts, cytosol, microsomal fraction, cell walls (1). Possible roles concern the structural modification of specific proteins. In the cytosol, they modify actin and tubulin, also influencing motor proteins, thus exerting a role on cell growth and cell structure (2). In chloroplasts, transglutaminases appear to stabilise the photosynthetic complexes and Rubisco, being regulated by light and other factors, and possibly exerting a positive effect on photosynthesis and photoprotection (1). Recent reports suggest an involvement in construction/organisation of the cell wall and extracellular matrix in relationship with fertilisation and a possible role of pollen extracellular TGase in cross-allergenicity. Other roles appear to be related to stresses, senescence and programmed cell death, including the hypersensitive reaction caused by TMV virus. The cross recognition of substrates between plant and animal enzymes suggest both similarities and differences. Proofs of their catalytic activity are: 1. their ability to produce glutamyl-polyamine derivatives, 2. their recognition by animal transglutaminase antibodies, 3. biochemical features such as calcium- and GTP-dependency, etc. 4. Inhibition by specific inhibitors of animal transglutaminases. However, many of their fundamental physiological properties still remain elusive. The few plant transglutaminases sequenced so far have little sequence homology with the best-known animal enzymes, except for the catalytic triad; however, they share a possible structural homology (3). Specificity and similarity with animal transglutaminases are discussed in the light of their biochemical characteristics and functional roles. 1. Serafini-Fracassini D and Del Duca S (2008) Tranglutaminases: widespread cross-linking enzymes in plants. Ann. Bot. 102, 145-152. DOI 10.1093/aob/mcn075. 2. Del Duca S. et al. (2008) Effects of post-translational modifications catalyzed by pollen transglutaminase on the functional properties of microtubules and actin filaments. Biochem. J., 418 3,651-664. 3. Serafini-Fracassini D, et al. (2008) Plant and animal transglutaminases: do similar functions imply similar structures? Amino Acids 36 (4) 643-657, DOI: 10.1007/s00726-008-0131-9 FUNDING: MUR-PRIN, Interaction mechanisms for protein mediators of flower incompatibility in fertilisation of fruit trees,to D.S-F;“Crossallergenicity”, Progetto Strategico d’Ateneo 2006 to D.S-F