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Drosophila Spire is an actin nucleation factor


The actin cytoskeleton is essential for many cellular functions including shape determination, intracellular transport and locomotion. Previous work has identified two factors—the Arp2/3 complex and the formin family of proteins—that nucleate new actin filaments via different mechanisms. Here we show that the Drosophila protein Spire represents a third class of actin nucleation factor. In vitro, Spire nucleates new filaments at a rate that is similar to that of the formin family of proteins but slower than in the activated Arp2/3 complex, and it remains associated with the slow-growing pointed end of the new filament. Spire contains a cluster of four WASP homology 2 (WH2) domains, each of which binds an actin monomer. Maximal nucleation activity requires all four WH2 domains along with an additional actin-binding motif, conserved among Spire proteins. Spire itself is conserved among metazoans and, together with the formin Cappuccino, is required for axis specification in oocytes and embryos, suggesting that multiple actin nucleation factors collaborate to construct essential cytoskeletal structures.

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Figure 1: Spir nucleates actin filaments.
Figure 2: Spir, Capu and the Arp2/3 complex nucleate polymerization by distinct mechanisms.
Figure 3: Molecular dissection of Spir.
Figure 4: Spir–actin complexes are rod-like structures.
Figure 5: Mechanism of nucleation by Spir.


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This work was supported by grants from the NIH, Pew Charitable Trust and the Sandler Family Supporting Foundation (to R.D.M.). E.K. was supported by the Deutsche Forschungsgemeinschaft and the Wilhelm Sander-Stiftung. We thank C. B. Leberfinger and J. M. Borawski for technical assistance, and members of the Mullins laboratory for technical help and for discussions.

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Correspondence to R. Dyche Mullins.

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This file contains the Supplementary Methods, Supplementary Data, Supplementary Figures S1-9 and Supplementary Table S1. It also includes additional references. (DOC 1925 kb)

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Quinlan, M., Heuser, J., Kerkhoff, E. et al. Drosophila Spire is an actin nucleation factor. Nature 433, 382–388 (2005).

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