Screen for enhancers of PKA in Drosophila oogenesis: The RNA binding protein Squid is required for anterior-posterior polarity

Protein Kinase A (PKA) is a ubiquitous serine/threonine kinase conserved amongst all eukaryotes. It has been implicated in a multitude of cellular processes, many of which are conserved from yeast to humans. PKA also plays developmental and physiological roles in multicellular organisms. A number of these roles have been elucidated in the fruit fly, Drosophila melanogaster. In characterizing the Drosophila homologue of the PKA catalytic subunit, PKA-C1, it was discovered that PKA plays an essential role in oogenesis.; The embryonic axes of the fly are established in the oocyte, via the localization of three key mRNA determinants in mid-oogenesis (stage 9). Localization of bicoid RNA to the anterior cortex and oskar RNA to the posterior cortex organizes the anterior-posterior body plan of the embryo, while localization of gurken RNA to the presumptive dorsal anterior corner of the oocyte specifies the dorsal-ventral axis of the eggshell and embryo. The localization of these three mRNAs depends on the microtubule (MT) organization within the oocyte. The MTs of the Drosophila germline are highly dynamic throughout oogenesis, and their precise coordination at stage 9 is believed to be critical for embryonic patterning. PKA is required for this coordination, but the mechanism of PKA function in this system is unknown.; In an effort to elucidate the mechanism of PKA function in oocyte MT organization, we performed a dominant enhancer screen of a PKA associated oogenesis polarity phenotype. The results of the screen are described in this work. We identified a single gene, squid, that encodes an RNA binding protein previously characterized as being required for dorsal-ventral axis induction during oogenesis. We find that Squid is essential also for anterior-posterior axis formation in the oocyte. We have extensively characterized the role of Squid in Drosophila oogenesis, and we show that it is required for the establishment of MT polarity within the germline at several stages of oocyte development. This function is germline specific, since loss of Squid in somatic cells does not disrupt their polarity. Thus, our work has identified a new component specifically required for MT polarity in the Drosophila oocyte.