Characterization of Calcium-Binding Light Chain as a Ca²⁺-Receptive Subunit of Physarum Myosin

Abstract

Physarum myosin is uniquely under an inhibitory Ca2+ -regulation in the ATP-dependent interaction with actin [Kohama (1990) Trends Pharmacol. Sci. 11, 433-435, for review]. Calcium-binding light chain (CaLc) has been suggested to be of primary importance to the control from its amino acid sequence [Kobayashi et al. (1988) J. Biol. Chem. 263, 305-313]. To provide a biochemical basis for this suggestion, the Ca-binding capacity of CaLc and its K_d for Ca²⁺ were measured. The Ca-binding properties of CaLc allowed those of Physarum myosin to be explained in terms of CaLc. However, the mode of Ca²⁺ -regulation by CaLc differs according to the enzyme upon which Ca-sensitivity is confered by CaLc, i.e., CaLc activated bovine phosphodiesterase activity and inhibited Physarum myosin ATPase activity, with the same K_d in μM levels. Thus, CaLc appears to work as a mere Ca-receptive subunit in Physarum myosin, with the secret of the inhibition lying in other subunits. CaLc was also shown to belong to a family of alkali light chains (AlLc) by allowing it to bind skeletal myosin as a substitute for its A1Lc. Therefore, present study is the first biochemical indication that the A1Lc family is involved in regulating the myosin function.