Interaction between lipids and bovine brain calmodulin: lysophosphatidylcholine-induced conformation change.

We have monitored the interaction of several lipids with the bovine brain calmodulin(CaM) and analyzed the effect of lysophosphatidylcholine(lyso-PC, 2-50 micrograms/ml) on conformation of CaM and the interaction between CaM and CaM-binding protein(CaMBP), using a fluorescence signal of 1-(dimethylamino)naphthalene-5-sulfonate-labeled CaM(DNS-CaM). Lyso-PC(egg, 20 micrograms/ml), among various natural lipids including phosphatidylserine(PS), phosphatidylinositol(PI), phosphatidylethanolamine (PE) and their lyso forms, greatly and dose-dependently enhanced the intensity of DNS fluorescence of DNS-CaM in the presence (100 microM CaCl2) and absence (1 mM EGTA) of Ca2+. Apparent dissociation constants calculated from the fluorometric titrations of binding of lyso-PC to DNS-CaM were 0.6 and 3.7 micrograms/ml in the presence and absence of Ca2+, respectively. Lyso-PC remarkably prevented both trypsin-induced quenching of the fluorescence of DNS-CaM and tryptic digestion of native CaM in the absence of Ca2+. Enhancement of DNS fluorescence of DNS-CaM by CaMBP was observed only in the presence of Ca2+ and lyso-PC could further increase the fluorescence intensity of the complex. These all results suggest that lyso-PC can modulate the interaction between CaM and CaMBP as a result of its direct effect on conformation of CaM.