UBIQUITIN-MEDIATED DEGRADATION OF TRYPTOPHAN DECARBOXYLASE FROM CATHARANTHUS-ROSEUS

Tryptophan decarboxylase (IDC) from Catharanthus roseus catalyses a highly regulated step in the biosynthesis of monoterpenoid indole alkaloids. Besides being transcriptionally regulated [Roewer, I. A. et al. (1992) Plant Cell Rep. 11, 86], the enzyme appears to be under post-translational control. Crude cell extracts from developing seedlings contain immunologically reactive proteins with molecular masses of 49 000, 54 800, 55 000, 63 000 and 68 000 which bind to anti-TDC IgG affinity columns. Each of these proteins is detected with anti-TDC antibodies on immunoblots, whereas only the 63 000 and 68 000 proteins are also detected with anti-ubiquitin antibodies. The addition of ATP to crude seedling extracts causes the rapid loss of TDC activity together with the appearance of proteolytically processed forms which react with anti-TDC antibodies on immunoblots. The appearance of these proteins is prevented by hemin, an inhibitor of ubiquitin-mediated proteolysis. The results suggest that ubiquitinated forms of TDC exist in vivo and that the ubiquitin proteolytic pathway may play an important role in the developmental regulation of TDC in Catharanthus roseus.