Novel mechanisms in actin polymerization: trans-regulation of the Spir-family proteins and Cappuccino-family formins

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The Drosophila spire and cappuccino mutants were identified in same genetic screen and have a nearly identical phenotype. The genes are required for the polarization of the Drosophila oocyte. The Spir and Cappuccino proteins belong to two distinct classes of actin organizers. The N-terminal region of the Spir protein shows sequence similarities to the Wiskott-Aldrich Syndrome protein (WASP), in that it encodes a cluster of four WH2 domains and an acidic region. The Cappuccino protein belongs to the formin protein family. Recently it has been shown that the yeast formin bni1p induces the formation of actin cables. Spir proteins colocalize with the Rab11 GTPase at the trans-Golgi network and post-Golgi vesicles and have a function in vesicle transport processes. The specific subcellular localization of the Spir proteins is mediated by a C-terminal FYCW zinc finger motif. In yet unpublished studies we have shown that both, Spir and Cappuccino, are localized at vesicle actin comet tails. This indicates a function of the proteins in actin dependent propulsion of cytoplasmic vesicles. In addition, we found both proteins highly phosphorylated in response to the activation of the p38 MAP kinase, which points to a common regulatory mechanism by MAP kinase signaling. Here we describe experiments to analyze the regulation and potential cooperation of the two actin organizers in the formation of vesicle actin comet tails.