The origin and composition of multiple forms of dextransucrase from Streptococcus sanguis

Multiple forms of purified dextransucrase have been observed in the presence of low detergent concentrations ( Luzio , G.A., Grahame , D. A. and Mayer, R.M. (1982) Arch. Biochem. Biophys. 216, 751-757). We now show these forms to arise partly as a result of proteolysis, and partly due to incomplete dissociation of the enzyme. Upon 25 degrees C incubation of the crude enzyme, several new bands appeared with little or no change in total activity. The electrophoretic pattern of aged, crude enzyme was similar to that of partially purified enzyme. Specific detection of dextransucrase on SDS gels revealed a single polypeptide of 174 kDa, which is converted to a 156 kDa protein during the aging process. The observation indicates the occurrence of proteolysis. The polypeptide composition of several of the enzyme forms was determined by two-dimensional electrophoresis. Forms Ia and IIa are composed exclusively of 174 kDa polypeptides. Forms III and IVa consist of 156 kDa units, as does the newly observed form Ic. It is likely that form Ib contains both 174 and 156 kDa polypeptides. The results indicate that incomplete dissociation of aggregates of the 174 kDa unit accounts for all of the bands observed on native gels run on fresh culture extracts. Additional enzyme forms result from aggregation of the 156 kDa proteolysis product alone, and from aggregation with unproteolyzed units to form hybrid aggregates.