Tetracycline/H+ antiporter was degraded rapidly in Escherichia coli cells when truncated at last transmembrane helix and this degradation was protected by overproduced GroEL/ES.

The in vivo degradation of the plasmid-encoded tetracycline/H+ antiporter (TET) in Escherichia coli cells was studied using three mutants with carboxyl-terminal truncation at the positions in the hydrophilic carboxyl-terminal tail (TET388), in the last putative transmembrane helix XII (TET382), and immediately before the helix XII (TET365). All the mutant TET proteins were localized in the membrane. Expressed TET388 was active in transport and stable against proteolysis. However, TET382 and TET365 were inactive and proteolyzed rapidly. Thus, the importance of the helix XII for protease-resistant proper folding of TET is obvious. Interestingly, overproduced chaperonin (GroEL and GroES) partly prevented degradation of TET365.