Identification of a region in segment 1 of gelsolin critical for actin binding.

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Abstract
The actin severing and capping protein gelsolin contains three distinct actin binding sites. The smallest actin binding domain of approximately 15,000 Mr was originally obtained by limited proteolysis and it corresponds to the first of six repeating segments contained in the gelsolin sequence. We have expressed this domain (here termed segment 1 or N150 to define its amino acid length) in Escherichia coli, together with a series of smaller mutants truncated at either N‐ or C‐terminal ends, in an attempt to localize residues critical of actin binding. Limited truncation of segment 1 by 11 residues at its N‐terminal end has no observable effect on actin binding, but on removal of a further eight residues, actin binding is totally eliminated. Although this loss of actin binding may reflect ablation of critical residues, we cannot rule out the possibility that removal of these residues adversely affects the folding of the polypeptide chain during renaturation. Truncation at the C‐terminus of segment 1 has a progressive effect on actin binding. Unlike intact segment 1, which shows no calcium sensitivity of actin binding within the resolution of our assays, a mutant with 19 residues deleted from its C‐terminus shows unchanged affinity for actin in the presence of calcium, but approximately 100‐fold weaker binding in its absence. Removal of an additional five residues from the C‐terminus produces a mutant that binds actin only in calcium. Further limited truncation results in progressively weaker calcium dependent binding and all binding is eliminated when a total of 29 residues has been removed. Although none of the expressed proteins on their own binds calcium, 45Ca is trapped in the complexes, including the complex between actin and segment 1 itself. These results highlight a region close to the C‐terminus of segment 1 that is essential for actin binding and demonstrate that calcium plays an important role in the high affinity actin binding by this domain of gelsolin.