Do transglutaminases have a role in the cell wall modification?

Do transglutaminases have a role in the cell wall modification? D. Serafini-Fracassini1, S. Del Duca1, R.A. Iorio1, G. Cai2, C. Faleri 2, M. Cresti 2 1 Dipartimento di Biologia, Università di Bologna, Via Irnerio 42, Bologna (Italy) 2 Dipartimento Scienze Ambientali, Università di Siena, via Mattioli 4, Siena (Italy) Transglutaminases (TGases) are a family of Ca2+-dependent enzymes, which appear to be widespread in all plant organs and cell compartments. They catalyze the post-translational conjugation of polyamines to different protein targets as well as the cross-link between the lysyl- and glutaminyl-residues of two proteins. Different plant models studied up to date seems to confirm that this enzyme is present in the cell wall of algae, fungi and higher plants, perhaps having a structural role similar to that exerted in the extracellular matrix of animal cells, namely matrix stabilisation, cell adhesion and cell migration. Some of these data will be presented starting from the first indirect evidence obtained by the digestion of Helianthus parenchyma cells by cellulase and pectinase which caused the disaggregation of high mass PA-conjugated proteins (Dinnella et al., 1992). A clear confirmation was obtained in isolated cell walls of Nicotiana petals, by detection of TGase immunoreactivity by western-blotting and catalytic activity (Della Mea et al., 2007) and by its immunodetection by confocal microscopy in the cell walls of corolla epidermis and, when the corolla undergoes senescence and become more rigid, also in those of mesophyll (Della Mea et al. in preparation). An extracellular form of transglutaminase was demonstrated to be involved in the apical growth of Malus domestica pollen tube. Transglutaminase-specific inhibitors and an anti-TGase monoclonal antibody blocked pollen tube growth. Apple pollen transglutaminase and its substrates, as determined by indirect immuno-fluorescence staining and the in situ cross-linking of fluorescently labeled substrates, were co-localized within aggregates on the pollen tube surface germinated both in vitro and in the style. These data are consistent with a role of TGase as modulator of cell wall building and strengthening and suggested a role for TGase in the interaction between pollen tubes and the extracellular matrix during fertilization (Di Sandro et al., 2010). In self-incompatible crosses, the activity of TGase is enhanced leading to the formation of high molecular mass cross-linked products around the pollen tube tip (questo non possiamo dirlo; possiamo dire solo che colocalizza nel plug di callosio…), as well as aggregates of tubulin and actin (Iorio et al. in press). Since actin filaments are perturbed during the self-incompatibility response, it is likely that the distribution and the activity of extracellular TGase is also affected, leading to the arrest of pollen tube growth. Using immunological probes, we have identified different TGase isoforms in association with different subcellular compartments (cytosol, membranes and cell wall). Cytosolic TGase binds to actin filaments in a Ca2+-dependent way (e la tubulin???). The membrane TGase is likely to be associated with both Golgi-derived structures and the plasma membrane. The distribution of cytoplasmic TGase is dependent on the cytoskeleton while a different TGase form is actively transported via a membrane/cytoskeleton-based transport system and is secreted in the cell wall of pollen tubes, suggesting a Golgi-based exocytosis.