Hydrophobic regions function in calmodulin-enzyme(s) interactions

Certain naturally occurring lipids (phosphatidylinositol, phosphatidylserine, arachidonic acid) and sodium dodecyl sulfate activate at least two calmodulin-dependent enzymes, bovine brain 3':5'-cyclic nucleotide phosphodiesterase and chicken gizzard myosin light chain kinase in the absence of Ca2+. 2-p-Toluidinyl-naphthalene-6-sulfonate (TNS), which is often used as a probe for hydrophobic groups of proteins, inhibits these two calmodulin-dependent enzymes. Kinetic analysis of inhibition of chicken gizzard myosin kinase by TNS revealed a competitive fashion against calmodulin-induced activation. The interaction between TNS and purified bovine brain calmodulin as demonstrated in the appearance of TNS fluorescence in the presence of 3 microM or more of calcium ion was not observed in the presence of 2 mM EGTA. This suggests that TNS is able to bind to calmodulin in the presence of Ca2+. Moreover, a calmodulin-interacting agent N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide suppressed the TNS fluorescence induced by complex formation with calmodulin in the presence of Ca2+. These results suggest that when Ca2+ binds to the high affinity sites of calmodulin, it induces a conformational change which exposes hydrophobic groups, and the calmodulin is then capable of activating calmodulin-dependent enzymes. We propose that hydrophobic properties of Ca2+-calmodulin are important for the activation of Ca2+-calmodulin-dependent enzymes.