Identification of fodrin as a major calmodulin-binding protein in postsynaptic density preparations

A major protein of postsynaptic densities (PSDs), a doublet of 230,000 and 235,000 Mr that becomes enriched in PSDs after treatment of synaptic membranes with 0.5% Triton X-100, has been found to be identical to fodrin (Levine, J., and M. Willard, 1981, J. Cell Biol. 90:631) by the following criteria. The upper bands of the PSD doublet and purified fodrin (alpha-fodrin) were found to be identical since both bands (a) co-migrated on SDS gels, (b) reacted with antifodrin, (c) bound calmodulin, and (d) had identical peptide maps after Staphylococcus aureus protease digestion. The lower bands of the PSD doublet and of purified fodrin (beta-fodrin) were found to be identical since both bands co-migrated on SDS gels and both had identical peptide maps after S. aureus protease digestion. The binding of calmodulin to alpha-fodrin was confirmed by cross-linking azido-125I-calmodulin to fodrin before running the protein on SDS gels. No binding of calmodulin to beta-fodrin was observed with either the gel overlay or azido- calmodulin techniques. A second calmodulin binding protein in the PSD has been found to be the proteolytic product of alpha-fodrin. This band (140,000 Mr), which can be created by treating fodrin with chymotrypsin, both binds calmodulin and reacts with antifodrin.