Structural studies on human spectrin. Comparison of subunits and fragmentation of native spectrin.

Native spectrin has trypsin-susceptible sites spaced at a constant molecular weight interval. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of spectrin treated with trypsin at low salt concentrations shows a ladder of fragments spaced at approximately 8,000-dalton intervals, from the intact Band 1 (240,000 daltons) and Band 2 (220,000 daltons) down to about 150,000 daltons. The five largest fragments were identified as products of Band 2 using tryptic 125I-peptide mapping of protein from gel slices. Endogenously incorporated [32P]phosphate is absent from the largest fragment, indicating that all phosphorylation sites on spectrin are within 8,000 daltons of a terminal of Band 2. Mapping of both [14C]carboxyamidomethylated cysteine-containing tryptic peptides and 125I-peptides reveals extensive sequence homology between the spectrin subunits. Further, only somewhat over half of the distinct spots expected from the cysteine content are found in both Band 1 and Band 2 peptides. These and the tryptic susceptibility results are interpretable as evidence for a repeating structure in spectrin.