Binding of an 80 000 dalton trypsin fragment of spectrin to intact spectrin.

A specific and saturable interaction of an 80 000 dalton tryptic fragment of spectrin with intact spectrin has been detected. When spectrin was incubated with 125I-labelled 80 kDa fragment at 37 degrees C in the presence of 0.1 M NaCl, acrylamide gradient gel electrophoresis showed the presence of bands in addition to the usual spectrin dimer and tetramer and the 80 kDa fragment. These bands correspond to 5.6 X 10(5) daltons (dimer + fragment), 1.04 X 10(6) daltons (tetramer + fragment) and 1.52 X 10(6) daltons (hexamer + fragment). Measurement of radioactivity showed that these additional bands contained the labelled fragment. Maximal binding capacity of the 80 kDa fragment was approximately 170 micrograms fragment per mg spectrin, corresponding to 1 mol fragment per mol spectrin dimer.