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Structure and function of the interacting domains of Spire and Fmn-family formins.

Proceedings of the National Academy of Sciences of the United States of America

metabolism, Actins, Animals, Crystallization, Drosophila Proteins, chemistry, genetics, Drosophila melanogaster, Fluorescence Polarization, Humans, Microfilament Proteins, Models, Molecular, Nerve Tissue Proteins, Oogenesis, physiology, Protein Conformation, Protein Structure, Tertiary

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      Evidence for cooperation between actin nucleators is growing. The WH2-containing nucleator Spire and the formin Cappuccino interact directly, and both are essential for assembly of an actin mesh during Drosophila oogenesis. Their interaction requires the kinase noncatalytic C-lobe domain (KIND) domain of Spire and the C-terminal tail of the formin. Here we describe the crystal structure of the KIND domain of human Spir1 alone and in complex with the tail of Fmn2, a mammalian ortholog of Cappuccino. The KIND domain is structurally similar to the C-lobe of protein kinases. The Fmn2 tail is coordinated in an acidic cleft at the base of the domain that appears to have evolved via deletion of a helix from the canonical kinase fold. Our functional analysis of Cappuccino reveals an unexpected requirement for its tail in actin assembly. In addition, we find that the KIND/tail interaction blocks nucleation by Cappuccino and promotes its displacement from filament barbed ends providing insight into possible modes of cooperation between Spire and Cappuccino.

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