Increase in the stability of serine acetyltransferase from Escherichia coli against cold inactivation and proteolysis by forming a bienzyme complex

Cysteine synthetase from Escherichia coli is a bienzyme complex composed of serine acetyltransferase (SAT) and O-acetylserine sulfhydrylase-A, (OASS), The effects of the complex formation on the stability of SAT against cold inactivation and proteolysis were investigated. SAT was reversibly inactiva ted on cooling to 0 degreesC. Ultracentrifugal analysis showed that SET (a hexamer) was dissociated mostly into two trimers on cooling to 0 degreesC i n the absence of OASS, while in the presence of BASS one trimer of the SAT subunits formed a complex with one dimer of BASS subunits. In the presence of BASS, trot only the cold inactivation rate was reduced but also the reac tivation rate was increased. Furthermore, SAT became stable against proteol ytic attack bg alpha -chymotrypsin and V8 protease by forming the complex w ith OASS, On the other hand, SAT was degraded by trypsin in the same manner both in the presence and in the absence of OASS. The different tendency in the stability against proteolysis with the different proteases was discuss ed with respect to the substrate specificity of the proteases and amino aci d sequence of the C-terminal region of SAT that interacts with OASS.