We previously identified an oocyte-specific gene, Oogenesin I (Oog1), that encodes 326 amino acids containing a leucine zipper structure and a leucine-rich repeat. In the present study, to identify the interacting proteins or Oog1, we performed a yeast two-hybrid screening using a GV-oocyte cDNA library and found that Ra1 guanine nucleotide dissociation stimulator (Ra1GDS) is the binding partner of Oog1. Coimmunoprecipitation assay confirmed the interaction between Oog1 and Ra1GDS proteins. Colocalization experiments provide the evidence that the nuclear localization of Ra1GDS depends on the expression of Oog1. Interestingly, Ra1GDS protein localized in the nucleus rather than the cytoplasm between late 1-cell and early 2-cell stages, the time when Oog1 localizes in the nucleus. We also examined the interaction between Oog1 and Ras by GST pull-down assay and revealed that Oog1 interacts with Ras in a GTP-dependent manner. These findings suggest a role of Oog1 as a Ras-binding protein. (c) 2006 Elsevier Inc. All rights reserved.