Circular dichroism studies of the Ca 2+-binding proteins from the spasmoneme of Carchesium

Abstract

1. 1. Ca 2+-binding proteins from Carchesium spasmoneme were purified previously, in this study the effects of Ca 2+ and Mg 2+ on the UV (210–250 nm) circular dichroism of several fractions were studied. 2. 2. Two fractions showed high affinity for Ca 2+ ( K m = 2.5 ∼ 4.5 μM), three others did not ( K m ≧ 20 μ M ). 3. 3. A purified fraction showed an increase in α-helical content with Ca 2+, but not with Mg 2+. 4. 4. The α-helical content increased with Ca 2+ with positive cooperativity ( n = 1.6), whereas the cooperativity decreased ( n = 1.2) in the presence of Mg 2+. 5. 5. The half-maximal change in α-helical content occurred at 4.5 μM free Ca 2+, which is close to the value obtained in the glycerinated stalk model of Vorticella.