Overlapping Functions of Argonaute Proteins in Patterning and Morphogenesis of Drosophila Embryos

Argonaute proteins are essential components of the molecular machinery that drives RNA silencing. In Drosophila, different members of the Argonaute family of proteins have been assigned to distinct RNA silencing pathways. While Ago1 is required for microRNA function, Ago2 is a crucial component of the RNA-induced silencing complex in siRNA-triggered RNA interference. Drosophila Ago2 contains an unusual amino-terminus with two types of imperfect glutamine-rich repeats (GRRs) of unknown function. Here we show that the GRRs of Ago2 are essential for the normal function of the protein. Alleles with reduced numbers of GRRs cause specific disruptions in two morphogenetic processes associated with the midblastula transition: membrane growth and microtubule-based organelle transport. These defects do not appear to result from disruption of siRNA-dependent processes but rather suggest an interference of the mutant Ago2 proteins in an Ago1-dependent pathway. Using loss-of-function alleles, we further demonstrate that Ago1 and Ago2 act in a partially redundant manner to control the expression of the segment-polarity gene wingless in the early embryo. Our findings argue against a strict separation of Ago1 and Ago2 functions and suggest that these proteins act in concert to control key steps of the midblastula transition and of segmental patterning.
Synopsis Cells employ diverse mechanisms to control the activity of their genes, and over the last ten years, a new strategy for gene regulation called RNA silencing has been discovered. Central components responsible for RNA silencing are Argonaute proteins. While many of the molecular properties of Argonaute proteins have been uncovered, little is known about their function in living organisms. In Drosophila, it has been puzzling that mutations in individual Argonaute proteins lead to surprisingly mild defects in development, although RNA silencing had been suggested to play major roles in gene regulation. Meyer and coworkers describe that Ago1 and Ago2, two Argonaute family members in Drosophila, function in a redundant fashion. Previously, these two proteins were demonstrated to mediate distinct pathways of RNA silencing. The authors show that in early embryos Ago1 and Ago2 work together in two fundamental processes: the generation of polarity within cells, by controlling the unequal distributions of proteins and cell organelles, and the polarity of tissues, by modulating an important cell-cell signaling pathway. These results connect the activity of Argonaute proteins and by extension the mechanisms of RNA silencing with central problems of cell and developmental biology, namely the regulation of polarity in cells and tissues.