Under standard conditions, G-actin has been submitted to nine proteases of varying specificity, and in each case the pattern of fragments produced has been studied by NaDodSO4 gel electrophoresis. The results suggest that the actin monomer consists of a large region (ca. 33 kilodaltons) and a small, easily degraded region (ca. 9 kilodaltons). The COOH terminus is in the large region. Consideration of primary sequence homologies, medium resolution maps of actin crystals, and certain reactions of actin suggests that the NH2 terminus is in the small region, as is the negative sequence to which a divalent metal cation is normally chelated, but that the nucleotide-binding site is on the large region near the junction between the regions. From analysis of these results, numerous properties of actin are understandable.
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