Bicaudal C and trailer hitch have similar roles in gurken mRNA localization and cytoskeletal organization

Bicaudal C and trailer hitch are both required for dorsoventral patterning of the Drosophila oocyte. Each mutant produces ventralized eggs, a phenotype typically associated with failure of the oocyte to provide a dorsalization signal – the Gurken protein – to the follicle cells. Bicaudal C and trailer hitch are both implicated in post-transcriptional gene regulation. Bicaudal C acts in recruiting a deadenylase to specific mRNAs, leading to translational repression. The role of trailer hitch is less well defined, but mutants have defects in protein secretion, and show aberrant distribution of an endoplasmic reticulum exit site marker whose mRNA is associated with Trailer hitch protein. We show that Bicaudal C and trailer hitch interact genetically. Mutants of these two genes have shared defects in localization of gurken and other anteriorly-localized mRNAs, as well as altered microtubule organization which may underlie the mRNA localization defects. Bicaudal C and trailer hitch mutants also share a syndrome of actin-related abnormalities, including the formation of ectopic actin cages near the anterior of the oocyte. The cages sequester Gurken protein, blocking its secretion and thus interfering with signaling of the follicle cells to specify dorsal fate.