Argonaute2, an essential component of the RISC, is a maternal factor required for membrane formation during cellularization. A.H. Mueller¹, S. Schreiber¹, H. Schwarz². 1) Institut fuer Genetik, Heinrich-Heine Universitaet, Duesseldorf, NRW, Germany; 2) Max-Planck-Institut fuer Entwicklungsbiologie, Tuebingen, Germany.

   Although the majority of gene products required for cellularization are provided maternally, only few maternal effect mutations have thus far been characterized which affect cellularization specifically. We demonstrate that the maternal effect mutation drop out (dop) blocks the early phase of membrane formation during cellularization. We show that embryos derived from dop mutant mothers lack typical basal adherens junctions during early cellularization. Furthermore, electron microscopy and immunological markers show that the generation of furrow canals is highly abnormal. Because these phenotypes are reminiscent of mutations of the zygotic factor slow as molasses (slam), we analyzed expression of slam in dop mutants. The expression-profiles of slam mRNA and protein are normal in dop mutants, suggesting that dop does not influence the transcription of slam or the translation of slam mRNA. Importantly, mutations in dop only affect the early phases of cellularization; the second phase of cellularization as well as further development occur rather normal. We have mapped and identified the gene affected by the dop mutation and show that it corresponds to argonaute2 (ago2). Ago2 constitutes an essential component of the RNA-induced silencing complex (RISC) in cell extracts from cultured cells (Hammond et al., 2001; Science 293, pp1146). Our results demonstrate a novel function for argonaute family proteins and provide an unexpected link between membrane formation and mechanisms involved in posttranscriptional gene silencing.