The phosphatidylethanolamine binding proteins (pebps) are an evolutionarily conserved family of proteins recently implicated in mitogen-activated protein (MAP) kinase pathway regulation, where they are called raf kinase inhibitory proteins. Here, we describe the cloning, cellular localization, and partial characterization of a new member, pebp-2, with potential roles in male fertility. Expression data show that pebp-2 is a testis-specific 21-kDa protein found within late meiotic and haploid germ cells in a stage-specific pattern that is temporally distinct from that of pebp-1. Sequence analyses suggest that pebp-2 forms a distinct subset of the pebp family within mammals. Database analyses revealed the existence of a third subset. Analysis suggests that the specificity/regulation of the distinct pebps subsets is likely to be determined by the amino terminal 40 amino acids or the 3' untranslated region, where the majority of sequence differences occur. Protein homology modeling suggests that pebp-2 protein is, however, topologically similar to other pebps and composed of Greek key fold motifs, a dominant beta-sheet formed from five anti-parallel beta strands forming a shallow groove associated with a putative phosphatidylethanolamine binding site. The pebp-2 gene is intronless and data suggest that it is a retrogene derived from pebp-1. Further, pebp-2 colocalizes with members of the MAP kinase pathway in late spermatocytes and spermatids and on the midpiece of epididymal sperm. These data raise the possibility that pebp-2 is a novel participant in the MAP kinase signaling pathway, with a role in spermatogenesis or posttesticular sperm maturation.

The sperm tail has a key role in male fertility, since it is responsible for motility, driving sperm toward and into the ovum. In this study, we characterized a sperm tail protein, testis specific protein 1 (Tpx-1), which is exclusively expressed in sperm and is localized to the acrosome and the outer dense fibres (ODF) of the sperm tail. We carried out yeast two-hybrid screening using an adult testis cDNA library to identify interacting protein partners of Tpx-1. A number of putative Tpx-1 interacting proteins were identified including a novel germ cell-specific protein, gametogenetin (Ggn). Ggn is highly expressed in the adult testis, specifically in late pachytene spermatocytes through to round spermatid germ cells. Ggn has more than 10 splice variants giving rise to three proteins namely GGN1, GGN2 and GGN3.1 The biological significance and potential for in vivo interactions between Tpx-1 and Ggn was assessed using Northern blot analysis, immuno-histochemistry and co-immunoprecipitation. To further investigate the nature of Tpx-1 interaction with Ggn, deletion studies were performed in yeast. The cysteine-rich carboxy terminal domain of Tpx-1 was shown to be responsible for binding a region in the last 120 amino acids of Ggn. Since the Ggn clone used for analysis encoded only this carboxy-terminal portion of the Ggn protein, it is also possible that other regions of the protein are involved in the interaction with Tpx-1. Further studies involving full-length clones and/or amino-terminal encoding Ggn clones will be needed to explore the possibility of additional interacting regions. In addition, Tpx-1 and Ggn knockout mice are also being generated. These results provide a greater understanding of the normal processes involved spermatogenesis and may suggest directed means to enhance or suppress male fertility. (1)Lu B, Bishop CE (2003). Mouse GGN1 and GGN3, two germ cell-specific proteins from the single gene Ggn, interact with mouse POG and play a role in spermatogenesis. J. Biol. Chem. 278, 16 289–16 296.