Ca2+-induced conformational changes of 20,000 dalton light chain of vertebrate striated muscle myosins

The 20,000 dalton light chain (L2) was isolated from rabbit and chicken striated muscle myosins, and the Ca2+-induced conformational changes of these proteins were investigated. 1) The reaction of thiol groups of L2 with dithiobisnitrobenzoic acid (DTNB), 2) measurements of the UV difference absorption spectrum, 3) measurements of Stokes radius (Rs) by gel filtration and 4) measurements of the ESR spectrum of L2 whose cysteine or tyrosine residues were spin-labeled were used for the structural studies. The effect of Ca2+ on phosphorylated L2 was also investigated. The long axis of chicken L2 was calculated as 136A from the Stokes radius, suggesting that the L2 is an asymmetrical molecule. After the addition of Ca2+ the long axis was reduced to 104 A. The same effect of Ca2+ has been reported with rabbit L2 (Alexis, N.M. & Gratzer, W.B. (1978) Biochemistry 17, 2319-2325). Besides this large shape change, the addition of Ca2+ to L2 induced environmental changes around tyrosine residues and also changes in the reactivity of cysteine residues with DTNB. Ca2+ is supposed to be bound to the N-terminal region of the molecule, while the tyrosine and cysteine residues are located at the C-terminal region, which is probably sterically remote from the N-terminal region. The reason for the remote effect of Ca2+ may be related to the structural rigidity of the L2 molecule. The functions of two properties of L2, Ca2+-binding and phosphorylation, are discussed in relation to muscle contraction.