Immature cells of etiolated apices of sprouts growing from Helianthus tuberosus (H. t.) tubers showed Ca(2+)-dependent transglutaminase (TG, EC 2.3.2.13) activity on fibronectin (more efficiently) and dimethylcasein as substrates. Three main TG bands of about 85, 75 and 58 kDa were isolated from the 100,000×g apices supernatant through a DEAE-cellulose column at increasing NaCl concentrations and immuno-identified by anti-TG K and anti-rat prostate gland TG antibodies. These three fractions had catalytic activity as catalyzed polyamine conjugation to N-benzyloxycarbonyl-L-γ-glutaminyl-L-leucine (Z-L-Gln-L-Leu) and the corresponding glutamyl-derivatives were identified. The amino acid composition of these TG proteins was compared with those of several sequenced TGs of different origin. The composition of the two larger bands presented great similarities with annotated TGs; in particular, the 75 kDa form was very similar to mammalian inactive EPB42. The 58 kDa form shared a low similarity with other TGs, including a maize sequence of similar molecular mass, which, however, did not present the catalytic triad in the position of all annotated TGs. A 3D model of the H. t. TGs was built adopting TG2 as template. These novel plant TGs are hypothesized to be constitutive and discussed in relation to their possible roles in immature cells. These data suggest that in plants, multiple TG forms are active in the same organ and that plant and animal enzymes probably are very close not only for their catalytic activity but also structurally.