A series of point mutations reveal interactions between the calcium-binding sites of calmodulin.
Calmodulin is a member of the "EF-hand" family of Ca(2+)-binding proteins. It consists of two homologous globular domains, each containing two helix-loop-helix Ca(2+)-binding sites. To examine the contribution of individual Ca(2+)-binding sites to the Ca(2+)-binding properties of CaM, a series of four site-directed mutants has been studied. In each, the glutamic acid at position 12 in one of the four Ca(2+)-binding loops has been changed to a glutamine. One-dimensional 1H-NMR has been used to monitor Ca(2+)-induced changes in the mutant proteins, and the spectral changes observed for each mutant have been compared to those for wild-type CaM. In this way, the effect of each mutation on both the mutated site and the other Ca(2+)-binding sites has been examined. The mutation of glutamate to glutamine at position 12 in any of the EF-hand Ca(2+)-binding loops greatly decreases the Ca(2+)-binding affinity at that site, yet differs in the overall effects on Ca2+ binding depending on which of the four sites is mutated. When the mutation is in site I, there is only a small decrease in the apparent Ca(2+)-binding affinity of site II, and vice versa. Mutation in either site III or IV results in a large decrease in the apparent Ca(2+)-binding affinities of the partner C-terminal site. In both the N- and C-terminal domains, evidence for altered conformational effects in the partners of mutated sites is presented. In the C-terminus, the conformational consequences of mutating site III or site IV are strikingly different.
Starovasnik
M A
MA
Department of Biochemistry, University of Washington, Seattle 98195.
Su
D R
DR
Beckingham
K
K
Klevit
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RE
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000235:genetics* / Q000378:metabolism*; D004252:DNA Mutational Analysis; D004331:Drosophila melanogaster / Q000235:genetics*; D005971:Glutamates / Q000235:genetics; D018698:Glutamic Acid; D005973:Glutamine / Q000235:genetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D017186:Titrimetry
D002147:Calmodulin; D005971:Glutamates; D005973:Glutamine; D018698:Glutamic Acid; D002118:Calcium