Formins are involved in a wide range of cellular processes that require the remodeling of the actin cytoskeleton. Here, we have analyzed a novel Drosophila formin, belonging to the recently described DAAM subfamily. In contrast to previous assumptions, we show that DAAM plays no essential role in planar cell polarity signaling, but it has striking requirements in organizing apical actin cables that define the taenidial fold pattern of the tracheal cuticle. These observations provide evidence the first time that the function of the taenidial organization is to prevent the collapse of the tracheal tubes. Our results indicate that although DAAM is regulated by RhoA, it functions upstream or parallel to the non-receptor tyrosine kinases Src42A and Tec29 to organize the actin cytoskeleton and to determine the cuticle pattern of the Drosophila respiratory system.

In the work reported here we have undertaken a functional dissection of a Polycomb response element (PRE) from the iab-7 cis-regulatory domain of the Drosophila melanogaster bithorax complex (BX-C). Previous studies mapped the iab-7 PRE to an 860-bp fragment located just distal to the Fab-7 boundary. Located within this fragment is an~230-bp chromatin-specific nuclease-hypersensitive region called HS3. We have shown that HS3 is capable of functioning as a Polycomb-dependent silencer in vivo, inducing pairing-dependent silencing of a mini-white reporter. The HS3 sequence contains consensus binding sites for the GAGA factor, a protein implicated in the formation of nucleosome-free regions of chromatin, and Pleiohomeotic (Pho), a Polycomb group protein that is related to the mammalian transcription factor YY1. We show that GAGA and Pho interact with these sequences in vitro and that the consensus binding sites for the two proteins are critical for the silencing activity of the iab-7 PRE in vivo.Segment identity in the posterior two-thirds of the Drosophila melanogaster embryo, from parasegment 5 (PS5) to PS14, is determined by the pattern of expression of the bithorax complex (BX-C) homeotic genes, Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) (13,32,39,46). These three homeotic genes are regulated by an elaborate cis-regulatory region that spans a DNA segment of over 300 kb. This large cis-regulatory region is subdivided into nine functionally autonomous domains, abx/bx, bxd/pbx, and iab-2 to iab-8 (2, 9, 29, 32). Each domain specifies the identity of a specific parasegment by activating one of the BX-C homeotic genes in a pattern appropriate for that parasegment. For example, the iab-5 cis-regulatory domain regulates Abd-B expression in a pattern that confers PS10 identity to the cells in this parasegment. Similarly, the iab-6, iab-7, and iab-8 cis-regulatory domains activate Abd-B expression in patterns appropriate for PS11, PS12, and PS13 identity, respectively (5, 9, 45). When one of the BX-C cis-regulatory domains is inactivated, the parasegment specified by the affected regulatory domain is transformed into a copy of the parasegment immediately anterior. Thus, in a deletion that inactivates iab-7, iab-7 Sz , PS12 is transformed into a duplicate copy of PS11 (16). In this case, Abd-B expression in both PS11 and PS12 is driven by the iab-6 cis-regulatory domain.

The regulation of growth cone actin dynamics is a critical aspect of axonal growth control. Among the proteins that are directly involved in the regulation of actin dynamics, actin nucleation factors play a pivotal role by promoting the formation of novel actin filaments. However, the essential nucleation factors in developing neurons have so far not been clearly identified. Here, we show expression data, and use true loss-of-function analysis and targeted expression of activated constructs to demonstrate that the Drosophila formin DAAM plays a critical role in axonal morphogenesis. In agreement with this finding, we show that dDAAM is required for filopodia formation at axonal growth cones. Our genetic interaction, immunoprecipitation and protein localization studies argue that dDAAM acts in concert with Rac GTPases, Profilin and Enabled during axonal growth regulation. We also show that mouse Daam1 rescues the CNS defects observed in dDAAM mutant flies to a high degree, and vice versa, that Drosophila DAAM induces the formation of neurite-like protrusions when expressed in mouse P19 cells, strongly suggesting that the function of DAAM in developing neurons has been conserved during evolution.