The plant extracellular transglutaminase: what mammal analogues tell

The extracellular transglutaminases (TGs) in eukaryotes are responsible for the post-translational modification of proteins through different reactions, cross-linking being the best known. In higher plants, extracellular TG appears to be involved in roles similar to those performed by the mammalian counterparties. Since TGs are pleiotropic enzymes, to fully understand the role of plant enzymes it is possible to compare them with animal TGs, the most studied being TG of type 2 (TG2). The extracellular form of TG2 stabilizes the matrix and modulates the interaction of the integrin-fibronectin receptor, causing the adhesion of cells to the extracellular matrix; TG2 plays a role also in the pathogenicity. Extracellular TGs have also been identified in the cell wall of fungi, such as Candida and Saccharomyces, where they cross-link structural glycoproteins, and in Phytophthora, where they are involved in pathogenicity; in the alga Chlamydomonas, TGs link polyamines to glycoproteins thereby favouring the strengthening of cell wall. In higher plants, TG localized in the cell wall of flower petals appears to be involved in the structural reinforcement as well as senescence and cell death of the flower corolla. In the pollen cell wall an extracellular TG co-localizes with substrates and cross-linked products; it is required for the apical growth of pollen tubes. The pollen TG is also secreted into the extracellular matrix possibly allowing the migration of pollen tubes during fertilisation. Although pollen TGs seem to be secreted via vesicles transported along actin filaments, a different mechanism from the classical ER-Golgi pathway is possible, similar to TG2.