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Oligomeric and polymeric protein associations often involve purely secondary chemical forces that is, hydrogen bonds, ionic bonds and van der Waals forces. Protein assembly and disassembly can also be regulated by post-translational modifications, such as phosphorylation or dephosphorylation, even in these cases the subunits are not linked covalently. The first transglutaminase enzyme (TGase, EC was recognised on the basis of its ability to catalyse the covalent incorporation of amine into proteins. In general, TGases are a family of enzymes that catalyse the covalent binding of substrates with primary amine groups to the carboxamide group of protein glutamine residues. These enzymes are called 'biological glues' in that they catalyse the post-translational modification of proteins by forming stable intra- and inter-molecular bridges. Through a similar catalytic process, transglutaminases can carry out acylation, esterification, deamidation and isopeptide cleavage, although the biological relevance of these reactions is characteristic only for animal cell and is less clear. Biological functions of TGases are generally attributed to their proteinmodifying activity, in some instances it is due to specialised non-catalytic actions, such as scaffolding of the cytoskeleton to maintain membrane integrity, cell adhesion and possibly signal transduction. Three structurally-characterised families of TGases have been identified. These are the papain-like TGases, members of the superfamily of cysteine proteases. These enzymes have a catalytic triad of Cys-His-Asp/Asn. Second group are the protein disulphide isomerase-like TGases, which have disulphide-bond isomerase activity in addition to their TGase activity, and third group is the bacterial toxin TGases; these proteins have an atypical catalytic triad and show no sequence homology with other two groups. Nine different TGase genes have been identified in the human genome. Eight encode potential Ca2+ regulated crosslinking enzymes and one encodes the catalytically inactive homologue. Apparent orthologues of the different human TGases have also been identified in organisms ranging from mammals to invertebrates. TGases are encoded by a family of closely related genes. All of the genes isolated so far seem to be organised in a similar way. Despite marked similarities in the organization of the TGase genes, their 5` flanking sequences and mechanisms of transcriptional regulation are not homologous, and this is consistent with their varied and regulated tissue-specific and developmental expression. High sequence conservation and high degree of preservation of residue of secondary structure indicates that all TGases might share a four-domain tertiary structure. Transglutaminases are Ca2+-dependent enzymes, as shown by EGTA inhibition. It has been demonstrated in pea root and leaf that calcium concentration affected the type of linkage that regulated intracellular role of TGases. High concentration of Ca2+ activated the formation of the glutamyl-lysyl isodipeptide bonds and inhibited amines conjugation to proteins. Other then Ca2+ factors like pH, magnesium and -SH protein groups can also modulate the enzyme activity. In animals transglutaminases are located both: intra- (cytosol, mitochondria, nucleus) and extra-cellularly in the matrix where they are involved in differentiation, transmembrane signalling, cell adhesion and organization of the extracellular matrix. Moreover, the presence and the activity of transglutaminases in dying animal cells are considered markers of apoptosis. Transglutaminases are also widespread in all plant organs and cell compartments. Much less is known about plant transglutaminases compared to their animal counterparts. In plants they are found at several different subcellular compartments, including the cytosol, cell wall, chloroplasts and mitochondria. Most data concern the chloroplast transglutaminase activity, which is regulated by light and known to modify Rubisco and several antenna proteins of photosystems I and II, influencing possibly the catalytic activity of the former and the energy transfer efficiency of the latter. Additional roles specific for plants are related to fertilisation, stresses, senescence and programmed cell death. AtPng1p, the first plant transglutaminase sequenced shows undetectable sequence homology to the animal enzymes, except for the catalytic triad (Cys-His-Asp). Despite that, AtPng1p shares with them immunological and biochemical properties and possibly a similar conformation. This review summarises our current knowledge of the structure, biochemical features, and cell localisation of animal and plant transglu- taminases and their biological role in the cell.

The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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