Overexpression of NtUBQ2 encoding Ub-extension protein enhances cadmium tolerance by activating 20S and 26S proteasome in tobacco (Nicotiana tabacum)

The Ub/26S proteasome system removes abnormal proteins and most short-lived regulatory proteins, thereby contributing to cell proliferation, hormone responses, development and resistance to abiotic and biotic stresses. Here we show that cadmium tolerance is related positively to the 20S proteasome (catalytic particle of 26S proteasome) activity in plants. By transforming WT yeast Y800 with a tobacco expression cDNA library, we isolated a tobacco cDNAs, NtUBQ2 (the Ub-extension protein) conferring cadmium tolerance. Overexpression of NtUBQ2 increased cadmium tolerance in transgenic tobacco; 20S proteasome activity was enhanced and ubiquitinated protein level was diminished in response to cadmium. In contrast, proteasome activity was reduced and ubiquitinated protein level was less decreased than transgenic tobacco by Cd treatment in control tobacco which is sensitive to Cd. These observations strongly suggest that plants acquire cadmium tolerance by removing cadmium-damaged proteins via Ub/26S proteasome-dependent proteolysis or Ub-independent 20S proteasome. This finding could be applied to engineering efficient metal phytoremediators.