Escherichia coli BL21 (DE3) is commonly used for the overproduction of fusion 
proteins. Using this system, we recently reported the overproduction of 
histidine-tagged mouse estrogen receptor (ER) alpha-ligand binding domain as an 
intact 30 kD protein and its inhibitory effect on the growth of bacteria. 
However, when GST-tagged mouse ERalpha transactivation domain (TAD) was 
overproduced using this system, it showed no effect on the growth of bacteria 
but was specifically degraded during its expression and purification. Here we 
report the expression of 47 kD GST-tagged mouse ERalpha-TAD protein, which was 
degraded partially and specifically into 46 and 43 kD fragments. This fusion 
protein was further degraded into 37, 31, 29 and 26 kD fragments during its 
purification by affinity chromatography. Such specific degradation of GST-tagged 
mouse ERalpha-TAD during its overproduction in E. coli and purification 
indicates the induction of specific protease and suggests the modification of 
expression system.