Germ cells carry out the reproductive function of the multicellular organisms, and normal germ cell development ensures survival of the species. Germ granules are conserved cytoplasmic organelles of the germ cells, essential for the survival, differentiation, and function of these cells. Mutations in germ granule components or loss of their expression lead to infertility in model organisms and are associated with infertilityin humans. By contrast, inappropriate expression of germ granule components in somatic cells is linked to carcinogenesis in humans. Many RNA-binding proteins and developmentally regulated mRNAs are found enriched in the germ granules, leading to a hypothesis that these organelles function in regulation of mRNA stability or translational activity; yet, the molecular function of these organelles is still undefined. The nematode C. elegans has been instrumental for understanding translational regulation of germline development. Our previous studies demonstrated specific contribution of C. elegans germ granules (P granules) to the regulation exerted by an RNA-binding regulatory protein FBF-2 in germline stem cells, yet much remains to be learned about the mechanistic basics of this contribution. Our experimental system is poised to address this question in molecular detail. Our studies will focus on FBF-2 as a paradigm of germ granule contribution to regulating the activity of RNA-binding proteins in germline. By integrating biochemical, molecular, genetic, and imaging-based approaches, we will: 1) Define the specific components of the FBF-2 regulatory complex; 2) Identify the cofactors of FBF-2 that depend on P granule integrity for their assembly with FBF-2; 3) Determine the sequences of FBF-2 mediating P granule recruitment and FBF-2-specific regulatory activity. These studies will reveal general mechanisms of germ granule-dependent regulation. Since the germ granules are conserved organelles and FBF-2 is a member of conserved PUF protein family, studies in this model system will provide critical insight into the causes of infertility i humans.