Twenty years of plant transglutaminases. Specificity and similarity with animal transglutaminases.

Transglutaminases have been studied in plants since 1987 in investigations aimed at interpreting some of the molecular mechanisms by which polyamines affect plant growth and differentiation. Among the myriad of biological reactions exerted in animals, the post-translational modification of proteins by polyamines forming inter- or intra-molecular cross-links has been the main transglutaminase reaction studied in plants. Transglutaminase activity is ubiquitous: it has been detected in algae and in angiosperms in different organs and sub-cellular compartments, chloroplasts being the best-studied organelles. Specificity and similarity with animal transglutaminases are discussed in the light of their biochemical characteristics and functional roles. 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. 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. Preliminary reports suggest an involvement in construction/organisation of the cell wall and extracellular matrix in relationship with fertilisation. 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. 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. 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. At present, it is not possible to classify this enzyme family in plants owing to the scarcity of information on genes encoding them.