Molecular interactions that regulate cystoblast differentiation. N.T. Minor , D.M. McKearin. Department of Molecular Biology,UT Southwestern Medical Center, Dallas, TX.

   A common feature of multicellular organisms is the need to replace cells in continuously cycling tissues; new cells are born to replace cells that die and are eliminated. Drosophila oogenesis is sustained by germline stem cells (GSCs) that occypy a niche at the anterior of the ovary. When GSC division produces two daugher cells, the one destined for the Cb fate begins to express the Cb program by escaping a TGFb-enriched microenvironment. Two of the essential cystoblast proteins, Bam and Bgcn, are the subjects of the present study.
   Bam is necessary and sufficient for the Cb fate. However, Bam is a novel protein, preventing functional insights from sequence comparisons. The role of Bam as a Cb differentiation factor probably involves a germ cell specific organelle, the fusome, which is composed of structural cytoskeletal proteins and ER-like membranes.
   Bgcn is necessary but insufficient for Cb formation. Bgcn is also an unusual protein, but it is distantly related to RNA-dependent helicases. The similarity of the bam and bgcn phenotypes and the expression pattern of these two genes suggested that these proteins might interact. This idea has been confirmed by a two-hybrid interaction between Bam and Bgcn.
   As part of the effort to discover how Bam and Bgcn act to control the swithc from GSC to Cb, we have undertaken a yeast two-hybrid screen with Bgcn as bait. The inital screen with Bgcn has yielded a-Spectrin, which is a major fusome component. In addition, a-Spectrin subcloned from the two-hybrid vector into a GST fusion vector produced a protein that can "pulldown" Bgcn-V5. These findings indicate that Bgcn probably resides at the fusome. We will also present data from a screen for genetic modifiers that we have initiated to complement the two-hybrid approach.