Calmodulin, a ganglioside-binding protein. Binding of gangliosides to calmodulin in the presence of calcium.

Abstract

Ca(2+)-dependent ganglioside-binding protein was isolated from a soluble cytosol fraction of mouse brains using a ganglioside affinity column prepared with a mixture of bovine brain gangliosides. It was identified as calmodulin based on the following features identical with those of calmodulin: molecular weight, pI, chromatographic profile and amino acid sequences of lysyl-endopeptidase digests, and ability to activate cyclic nucleotide phosphodiesterase. Bovine brain calmodulin derivatized with 5-dimethylaminonaphthalene-1-sulfonyl (dansyl-calmodulin), tetramethylrhodamine isothiocyanate, or biotin was also shown to bind to the ganglioside affinity column Ca2+ dependently and elute with gangliosides GD1a, GD1b, GT1b, GQ1b, GM1, and GM2, melittin, and trifluoperazine but not with GgOse4Cer and oligosaccharides of GM1, GD1a, and GT1b. Modification of the Lys94 residue of calmodulin by biotinylation drastically reduced the capacity for ganglioside binding. Ganglioside GD1b caused a blue shift and increase in intensity of the fluorescence emission spectrum of dansyl-calmodulin in the presence of Ca2+. The increment in fluorescence was proportional to the amount of GD1b added and was maximal at the molar ratio of GD1b to calmodulin, approximately 7.8. Gangliosides are thus shown to specifically bind to calmodulin, and this binding may be a general mechanism for regulating calmodulin-dependent enzymes with consequent cellular response, such as cell differentiation.