Restoration of the calcium binding activity of mutant calmodulins toward normal by the presence of a calmodulin binding structure

The altered calcium binding activity of calmodulins (CaM) with point mutations can be restored toward that of wild type CaMs by the formation of a complex between CaM and a CaM binding sequence. Three different site-specific mutations resulted in selective effects on the apparent stoichiometry and affinity of CaM for calcium, with maintenance of the ability to activate myosin light chain kinase. The effects on calcium binding, however, were suppressed when the mutant CaMs were complexed with RS20, a peptide analog of a myosin light chain kinase CaM binding site. The mutations included: 1) a Glu----Ala mutation at two phylogenetically conserved calcium ligands in the second (E67A-CaM) and fourth (E140A-CaM) sites; and 2) a Ser----Phe mutation at residue 101 (S101F-CaM) which affects ion channel regulation. The mutant CaMs bind 4 calciums in the absence of magnesium, but two sites have approximately 60- to 300-fold weaker binding than wild-type CaM (SYNCAM CaM). E67A-CaM and E140A-CaM bound only two calciums and S101F-CaM bound 4 calciums in the presence of magnesium. E67A-CaM and E140A-CaM recovered the ability to bind 4 calcium ions in the presence of the RS20 CaM binding peptide. These results are consistent with models in which the calcium binding activity of CaM within a supramolecular complex is different from purified CaM and raise the possibility that the selective functional effects of in vivo mutations in the calcium binding sites of CaM might be partially due to the ability of some CaM binding proteins to select and utilize CaM conformations with calcium ligation structures different from the so-called canonical EF-hand.