Subunit Stoichiometry of the Clostridium botulinum Type A Neurotoxin Complex Determined Using Denaturing Capillary Electrophoresis

A denaturing capillary electrophoresis method was developed to evaluate the subunit stoichiometry of the Clostridium botulinum type A neurotoxin complex. The results indicate that the neurotoxin complex contains single copies of the 150 kDa neurotoxin and the non-toxic non-hemagglutinating subunits as well as 5-6 HA17, 4-5 HA23, 3-4 HA48, and 8-9 HA34 subunits. The calculated molecular mass for a complex with this stoichiometry is between 880 and 1,000 kDa. The molecular mass of the intact complex was determined using size-exclusion HPLC (SE-HPLC) and SE-HPLC in conjunction with multi-angle laser light scattering detection. Based on a comparison to a mixture of standard proteins, SE-HPLC analysis yielded a molecular mass of 880 kDa while light scattering analysis indicated a weight average molecular mass of 925 +/- 45 kDa. The close agreement between the molecular mass values determined by the three approaches supports the subunit stoichiometry proposed for the C. botulinum type A neurotoxin complex.