Proteomic profiling of accessory structures from the mouse sperm flagellum

The flagellum of a mammalian spermatozoon consists of an axoneme surrounded in distinct regions by accessory structures known as the fibrous sheath, outer dense fibers, and the mitochondrial sheath. Although the characterization of individual proteins has provided clues about the roles of these accessory structures, a more complete understanding of flagellar function requires the identification of all the polypeptides in these assemblies. Epididymal mouse sperm were treated with SDS to dislodge sperm heads and to extract the axoneme and membranous elements. The remaining flagellar accessory structures were purified by sucrose gradient centrifugation. Analysis of proteins from these structures by two-dimensional gel electrophoresis and colloidal Coomassie Blue staining showed a highly reproducible pattern of >200 spots. Individual spots were picked, digested with trypsin, and identified by mass spectrometry and peptide microsequencing. Approximately 50 individual proteins were identified that could be assigned to five general categories: 1) proteins previously reported to localize to the accessory structures, e.g. ODF2 in the outer dense fibers, the sperm-specific glyceraldehyde-3-phosphate dehydrogenase in the fibrous sheath, and glutathione peroxidase in the mitochondrial sheath, validating this proteomic approach; 2) proteins that had not been shown to localize to any accessory structure but would be predicted to be present, e.g. glycolytic enzymes; 3) proteins known to be part of the flagellum but not localized to a specific site, e.g. adenylate kinase; 4) proteins not expected to be part of the accessory structures based on their previously reported locations, e.g. tektins; and 5) unknown proteins for which no information is available to make a determination as to location. The unexpected presence of the tektins in the accessory structures of the flagellum was confirmed by both immunoblot and immunofluorescence analysis. This proteomic analysis identified a number of unexpected and novel proteins in the accessory structures of the mammalian flagellum.