The role of Bicoid Stability Factor in oskar regulation. Young Hee Ryu, Paul Macdonald. Molecular Cell & Developmental Biology, University of Texas at Austin, Austin, TX.

   Axial patterning of the Drosophila oocyte and embryo relies on asymmetric localization of body patterning determinants. One such determinant, the Oskar (Osk) protein that is responsible for posterior patterning and germ cell formation, accumulates only at the posterior pole of the oocyte. This restriction is achieved by a coordinated program of osk mRNA localization and translational regulation. Many of the cis-acting elements required for regulation lie in the osk mRNA 3 untranslated region (3 UTR). The osk mRNA also has a function in progression through oogenesis independent of its protein coding capacity. This osk RNA function is mediated by the 3' UTR. A short region near the 3 end of the osk mRNA contains both regulatory elements and osk RNA function elements. To identify proteins that bind to this region, and could act in regulation or RNA function, we used a streptavidin (S1) aptamer affinity purification strategy. Hybrid transcripts containing the osk 3 region, in either wild type or mutant forms, and the S1 aptamer were bound to streptavidin beads and mixed with ovary extracts to allow assembly of RNP complexes. Bound proteins were identified by mass spectrometry. Bicoid Stability Factor (BSF) was identified as a protein associated with the wild type RNA but not a subset of mutant RNAs. Like osk RNA null mutants, bsf mutants are arrested in oogenesis at an early stage. However, the phenotypes of the bsf and osk RNA null mutants are not identical. Moreover, not all of the osk RNA 3' region mutants that fail to bind BSF are defective in osk RNA function. Therefore, BSF appears not to act in osk RNA function. Reducing bsf activity late in oogenesis interferes with Osk protein accumulation in early embryos, as does an osk RNA 3' region mutant that fails to bind BSF. These results argue that BSF has a role in regulation of Osk protein expression, either directly affecting translation or indirectly reducing Osk accumulation through an effect on osk mRNA stability or localization.