Multiple roles of the F-box protein Slimb inDrosophilaegg chamber development

Abstract

Substrate-specific degradation of proteins by the ubiquitinproteasome pathway is a precise mechanism that controls the abundance of key cell regulators. SCF complexes are a family of E3 ubiquitin ligases that target specific proteins for destruction at the 26S-proteasome. These complexes are composed of three constant polypeptides -Skp1, Cullin1/3 and Roc1/Rbx1 -and a fourth variable adapter, the F-box protein. Slimb (Slmb) is a Drosophila F-Box protein that fulfills several roles in development and cell physiology. We analyzed its participation in egg chamber development and found that slmb is required in both the follicle cells and the germline at different stages of oogenesis. We observed that in slmb somatic clones, morphogenesis of the germarium and encapsulation of the cyst were altered, giving rise to egg chambers with extra germline cells and two oocytes. Furthermore, in slmb somatic clones, we observed ectopic Fasciclin 3 expression, suggesting a delay in follicle cell differentiation, which correlated with the occurrence of ectopic polar cells, lack of interfollicular stalks and mislocalization of the oocyte. Later in oogenesis, Slmb was required in somatic cells to specify the position, size and morphology of dorsal appendages. Mild overactivation of the Dpp pathway caused similar phenotypes that could be antagonized by simultaneous overexpression of Slmb, suggesting that Slmb might normally downregulate the Dpp pathway in follicle cells. Indeed, ectopic expression of a dad-LacZ enhancer trap revealed that the Dpp pathway was upregulated in slmb somatic clones and, consistent with this, ectopic accumulation of the co-Smad protein, Medea, was recorded. By analyzing slmb germline clones, we found that loss of Slmb provoked a reduction in E2f2 and Dp levels, which correlated with misregulation of mitotic cycles during cyst formation, abnormal nurse cell endoreplication and impairment of dumping of the nurse cell content into the oocyte.

The role of Dpp and its inhibitors during eggshell patterning inDrosophila

“…1C,F). The same was observed for the distribution of the common Smad Medea (data not shown) (Wisotzkey et al, 1998;Parker et al, 2004;Muzzopappa and Wappner, 2005). We conclude that at least with regard to high and medium signaling levels (see below) there is no influence of Grk on Dpp signaling and primary Dpp target gene expression.…”

“…These cells secrete Dpp protein that forms an anterior-to-posterior gradient as suggested from the distribution of the Dpp signal transducer pMAD (Tanimoto et al, 2000) and the expression of the direct Dpp target gene, dad-lacZ (Tsuneizumi et al, 1997;Muzzopappa and Wappner, 2005) (Fig. 1A-F).…”

A Pak-regulated cell intercalation event leading to a novel radial cell polarity is involved in positioning of the follicle stem cell niche in theDrosophilaovary

“…Side-by-side cysts in the germarium have been described previously, but these are not individually encapsulated as they become egg chambers (Frydman and Spradling, 2001;Gonzalez-Reyes and St Johnston, 1998;Hartman et al, 2010;McCaffrey et al, 2006;Muzzopappa and Wappner, 2005;O'Reilly et al, 2006;Song and Xie, 2003). In most cases, these side-by-side cysts appear to be due to a failure of cysts to flatten at the 2a/2b boundary (Frydman and Spradling, 2001;Gonzalez-Reyes and St Johnston, 1998;McCaffrey et al, 2006;Muzzopappa and Wappner, 2005;O'Reilly et al, 2006). However, this was not the case in Pak mutant ovarioles, where side-by-side flattening of cysts occurred (Fig.…”

F-box proteins: the key to protein degradation

“…In the Dorsal/NFjB pathway, SCF Slimb is responsible for the degradation of Cactus/IjB [25]. In addition, Slimb is involved in the regulation of circadian rhythm in flies [26,27], plays multiple roles in Drosophila egg chamber development [34], and regulates the appearance of additional centrosomes during mitosis [35]. Another well-studied F-box gene, archipelago (ago), is implicated in the cell proliferation process.…”