Abstract
The chymotryptic digestion of G-actin in the presence of calcium produces not only a C-terminal 33 kDa “core, ” spanning from residue 68 to the C-terminus, but also an N-terminal Cys-lO-containing fragment (10 kDa fragment), spanning from the N-terminus to the 44 th residue. The minimum calcium concentration required for producing just these two structures is 10-7.5M. In a Ca medium, the 10 kDa fragment remains attached to the 33 kDa core, and the 10 kDa fragment detaches when the divalent cation is removed from the complex, as was proved by Sephacryl S-200 gel filtration. We conclude that 10 and 33 kDa form a complex that is calcium-sensitive. The Cys-10 in the 10 kDa moiety of the complex reacts with 5-iodoacetamide fluorescein in the presence of calcium ion, whereas Cys-257 is practically inert. The removal of calcium allows Cys-257 also to react with the reagent. Therefore, the complex seems to retain the calcium binding site. The nucleotide binding ability of the complex was also demonstrated.
