Testis-Specific RT7 Protein Localizes to the Sperm Tail and Associates with Itself1

The RT7 gene recently cloned by us is expressed as an abundant RNA in round spermatids. In vitro transcription-translation showed that the RT7 gene encodes a protein of 26-27 kDa on SDS-polyacrylamide gels. Here we report the development of monoclonal antibodies (mAbs) raised against a peptide from the predicted N-terminal amphipathic alpha-helix of the rat RT7 protein. All mAbs recognize RT7 protein or N-terminal parts of it. To investigate RT7 in vivo, mAbs were used in immunofluorescence microscopy and confocal laser immunofluorescence microscopy. Several mAbs recognize RT7 protein in elongating spermatids: the observed staining pattern suggests a nonrandom localization in these cells. Two mAbs recognize the protein only in sperm tails. Using co-immunoprecipitation assays, we found that RT7 can form stable complexes with itself that are associated through a region located in the N-terminal half of RT7. Our results identify the RT7 protein as a major sperm tail component and suggest that it may be a structural component of sperm tail outer dense fibers (ODF).