The anterior system is one of the four maternal systems for assuring proper polarity of the oocyte prior to fertilization. Genes affecting the localization of Bicoid belong to the anterior group. Bicoid is the principle protein for structuring the head and thorax of the developing fly, and Bicoid messenger RNA is transported to the anterior pole of the oocyte during oocyte development.

Localization of Bicoid mRNA is divided into three phases. In the previtellogenic stage, Bicoid mRNA is localized to the apex of each of the 15 nurse cells of the ovule. In vitellogenesis, the contents of the nurse cells are transported into the oocyte by a cytoskeletal based mechanism of cytoplasmic streaming. In a third process, Bicoid mRNA is transported along the microtubule network of the oocyte to its anterior pole. The latter process requires direction action of Exuperantia, Exuperantia-like, Swallow and Staufen, and involves binding of Exu-like to cis-acting localization signals within the 3' untranslated region of Bicoid mRNA. Staufen also binds to the 3' UTR of BCD mRNA.

The Drosophila eggshell is an extracellular matrix that confers protection to the egg and also plays a role in transferring positional information from the ovary to pattern the embryo. Among the constituents of the Drosophila eggshell, Nasrat, Polehole and Closca form a group of proteins related by sequence, secreted by the oocyte and mutually required for their incorporation into the eggshell. Besides their role in eggshell integrity, Nasrat, Polehole and Closca are also required for embryonic terminal patterning by anchoring or stabilizing Torso-like at the eggshell. This study shows that they are also required for dorsoventral patterning, thereby unveiling that the dorsoventral and terminal systems, hitherto considered independent, share a common extracellular step. Furthermore, Nasrat, Polehole and Closca are required for proper activity of Nudel, a protease acting both in embryonic dorsoventral patterning and eggshell integrity, thus providing a means to account for the role of Nasrat, Polehole and Closca. It is proposed that a Nasrat/Polehole/Closca complex acts as a multifunctional hub to anchor various proteins synthesized at oogenesis, ensuring their spatial and temporal restricted function (Mineo, 2017).

Terminal and dorsoventral signaling rely on initial spatial cues, which originate in the follicle cells surrounding the oocyte, that induce pattern formation in embryogenesis. Since follicle cells degenerate long before the cues perform their action in embryogenesis, all the information necessary for embryonic patterning has to be retained in the egg. In this scenario, the role of Nasrat, Polehole, and Closca in the localization of Tsl and Ndl suggests that a Nasrat/Polehole/Closca complex acts as a multifunctional hub at the vitelline membrane to anchor various proteins synthesized at oogenesis and with later functions in the eggshell and/or in triggering embryonic patterning (Mineo, 2017).

Although the mechanism responsible for eggshell integrity is not fully understood, Ndl, and in particular its protease activity, Nasrat, Polehole, and Closca clearly participate in this process. The current results now identify Ndl as an effector of Nasrat, Polehole, and Closca both in eggshell integrity and in their so far unknown role in dorsoventral patterning. In this regard, it is worth mentioning that, in spite of the many analyses of Ndl activity, it remains an open question as to whether its function in dorsoventral axis specification and eggshell integrity are independent of each other. Besides, LeMosy and collaborators have proposed an additional role for the nonprotease regions of Ndl in eggshell integrity (LeMosy, 2000; Mineo, 2017 and references therein).

Likewise, it is difficult to establish whether the diverse roles of Nasrat, Polehole, and Closca imply specific and independent functional protein domains. Although Nasrat, Polehole, and Closca belong to a common group of proteins, they show only moderate similarity, and no functional domains have been identified in any of them. The observation that a point mutation impairs the terminal functions of Clos proteins, as well as dorsoventral patterning and eggshell integrity, suggests a lack of clear independent domains responsible for each individual function. However, fs(1)N²¹¹ and fs(1)ph¹⁹⁰¹ mutants are thought to specifically impair the terminal function of Nasrat and Polehole proteins, respectively, which suggests that these might be modular proteins with different functional domains. Similarly, the fs(1)N^A1038 mutation supports the notion of independent functional domains. In particular, all eggs from homozygous fs(1)N^A1038 females collapse due to eggshell integrity defects; the same phenotype is observed in hemizygous fs(1)N^A1038 females and in transheterozygote females of fs(1)N^A1038 over a null fs(1)N mutant allele. However, eggs from transheterozygous females of fs(1)N^A1038 over the fs(1)N²¹¹ terminal allele give rise to wild-type larvae and adults. This intra-allelic complementation suggests that separate domains specifically affect the integrity and the terminal functions of the Nasrat protein. To further characterize these putative protein domains, this study mapped the molecular lesion in the fs(1)N^A1038 mutation and found it to correspond to an E to V transition at residue 350. This observation suggests that the domain of the Nasrat protein encompassing this residue is required for eggshell integrity but has no effect on embryonic patterning (Mineo, 2017).

In conclusion, this study has found that terminal and dorsoventral signaling, hitherto considered independent in their extracellular pathways, have Nasrat, Polehole, and Closca as common mediators. It is proposed that a complex of these proteins constitutes a multifunctional hub to ensure the proper temporal localization/stabilization and activity of proteins synthesized at oogenesis and required at egg activation, thus guaranteeing the coordination of the hardening of the eggshell with the trigger of early embryonic patterning (Mineo, 2017).