The sequence location of the actin metal

We have incorporated Fe²⁺ into the high‐affinity metal‐ion‐binding site of actin. By supplying the system with oxygen from air and a reductant (dithiothreitol or ascorbate), we have induced freeradical generation, with the intent of causing peptide cleavage at the metal‐ion‐binding site. By analysis of the resulting fragments from actin in the F‐form, we have deduced that cuts occurred at positions 159–160 and 301–302 (at the latter location we could not be sure if more than one cut occurred). We considered that these two cuts occurred in the chain strand coursing from the outer to the inner domain and vice‐versa. Our results harmonize very well with the recently reported atomic structure of actin [Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F. & Holmes, K. C. (1990) Nature 347, 37–44] and remove ambiguities that had remained in the structure. The results partly bear out the homology‐based prediction of Strzelecka‐Golaszewska et al. [Strzelecka‐Golaszewska, H., Boguta, G., Zmorzynshi, S. & Moraczcwska, J. (1989) Eur. J. Biochem. 182, 299–305].