Fig. 7: Model for TG2 activation by Ca²⁺ in the presence and absence of the inhibitor, LM11.

a When GTP is added to TG2 in buffer, it can effectively adopt the closed conformation even in the presence of Ca²⁺. In the absence of LM11, the addition of Ca²⁺ to TG2 is accompanied by the formation of a dimer on short time scales (300 ms to 5 min) and higher order oligomers at longer time scales (30 min). This process is reversible by the addition of excess GTP, through interactions at the dimer interface. The R580K mutant exists as a higher order oligomer in the absence of calcium and GTP, but transitions to the nucleotide bound open state dimer upon addition of excess GTP. The addition of Ca²⁺ to the R580K dimer is accompanied by the formation of higher order oligomers. b The presence of LM11 and Ca²⁺ maintains TG2 in an open conformation which is not reversed by the addition of GTP. Moreover, TG2 adopts a dimeric species that has a distinct conformation to the dimer fraction observed in the absence of LM11. Even on short time scales less than one second, higher-order oligomers are observed which are irreversible by the addition of excess GTP.