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The class I myosin genes are conserved in diverse organisms, and their gene products are involved in actin dynamics, endocytosis, and signal transduction. Drosophila melanogaster has three class I myosin genes, Myosin 31DF (Myo31DF), Myosin 61F (Myo61F), and Myosin 95E (Myo95E). Myo31DF, Myo61F, and Myo95E belong to the Myosin ID, Myosin IC, and Myosin IB families, respectively. Previous loss-of-function analyses of Myo31DF and Myo61F revealed important roles in left-right (LR) asymmetric development and enterocyte maintenance, respectively. However, it was difficult to elucidate their roles in vivo, because of potential redundant activities. Here we generated class I myosin double and triple mutants to address this issue. We found that the triple mutant was viable and fertile, indicating that all three class I myosins were dispensable for survival. A loss-of-function analysis revealed further that Myo31DF and Myo61F, but not Myo95E, had redundant functions in promoting the dextral LR asymmetric development of the male genitalia. Myo61F overexpression is known to antagonize the dextral activity of Myo31DF in various Drosophila organs. Thus, the LR-reversing activity of overexpressed Myo61F may not reflect its physiological function. The endogenous activity of Myo61F in promoting dextral LR asymmetric development was observed in the male genitalia, but not the embryonic gut, another LR asymmetric organ. Thus, Myo61F and Myo31DF, but not Myo95E, play tissue-specific, redundant roles in LR asymmetric development. Our studies also revealed differential colocalization of the class I myosins with filamentous (F)-actin in the brush border of intestinal enterocytes.KEYWORDS myosin I; Myosin 31DF; Myosin 61F; Myosin 95E; left-right asymmetry T HE class I myosin genes encode myosin heavy chains, which are conserved in phylogenetically diverse organisms (Sellers 2000;Krendel and Mooseker 2005). The class I myosins are nonfilamentous, actin-based motor proteins and were the first discovered unconventional myosin proteins. These myosins are involved in a variety of cellular processes, such as cell migration, cell adhesion, and cell growth, through their regulation of actin dynamics, endocytosis, and signal transduction (Osherov and May 2000;Krendel and Mooseker 2005;Kim and Flavell 2008;McConnell and Tyska 2010).The structure of the myosin I heavy chains is evolutionarily conserved and composed of head (or motor), neck, and tail domains ( Figure 1A) (Coluccio 1997;Barylko et al. 2000). The head domain binds to filamentous (F)-actin and adenosine triphosphate (ATP), a common feature of myosin proteins ( Figure 1A) (Mermall et al. 1998); the neck domain possesses one or more IQ motifs, which directly interact with calmodulin or calmodulin-related myosin light chains (Coluccio 1997;Barylko et al. 2000), and the tail domains are divided into short and long types. Short tails contain a single tail homology 1 (TH1) domain, which is rich in basic residues and thought to interact with plasma membranes (Coluccio 1997;Barylko...

SUMMARYThe Notch (N) signaling machinery is evolutionarily conserved and regulates a broad spectrum of cell-specification events, through local cell-cell communication. pecanex (pcx) encodes a multi-pass transmembrane protein of unknown function, widely found from Drosophila to humans. The zygotic and maternal loss of pcx in Drosophila causes a neurogenic phenotype (hyperplasia of the embryonic nervous system), suggesting that pcx might be involved in N signaling. Here, we established that Pcx is a component of the N-signaling pathway. Pcx was required upstream of the membrane-tethered and the nuclear forms of activated N, probably in N signal-receiving cells, suggesting that pcx is required prior to or during the activation of N. pcx overexpression revealed that Pcx resides in the endoplasmic reticulum (ER). Disruption of pcx function resulted in enlargement of the ER that was not attributable to the reduced N signaling activity. In addition, hyper-induction of the unfolded protein response (UPR) by the expression of activated Xbp1 or dominant-negative Heat shock protein cognate 3 suppressed the neurogenic phenotype and ER enlargement caused by the absence of pcx. A similar suppression of these phenotypes was induced by overexpression of O-fucosyltransferase 1, an N-specific chaperone. Taking these results together, we speculate that the reduction in N signaling in embryos lacking pcx function might be attributable to defective ER functions, which are compensated for by upregulation of the UPR and possibly by enhancement of N folding. Our results indicate that the ER plays a previously unrecognized role in N signaling and that this ER function depends on pcx activity.

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