The reactivity and function of thiol groups in trout actin

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Abstract
1. Considerable differences were found between the rates and degrees of modification of native trout actin with iodo[2-14C]acetate and iodo[1-14C]acetamide. 2. With iodoacetate, G- and F-actin were both labelled in the N-terminal peptide only. This modification had little effect on the ability of the actin to polymerize. 3. Iodoacetamide labelled three cysteine residues in both G- and F-actin. The modified cysteine residues were identified from the position of the corresponding tryptic peptides on peptide ‘maps’. 4. The modification had little effect on the ability of G-actin to polymerize, to bind ATP or to bind Ca2+, or on the ability of F-actin to depolymerize. 5. It is concluded that the three cysteine residues present on the ‘surface’ of the native trout actin molecule have no direct role in the polymerization processes, the binding of ATP, or the binding of Ca2+.