A commercial human protamine-2 antibody used in several studies to detect mouse protamine-2 recognizes mouse transition protein-2 but not protamine-2.

The exchange of histones for transition proteins (TNPs) and finally protamines is an essential process during spermatogenesis that enables the strong condensation of chromatin during sperm formation. Research on this process obviously depends on the availability of specific antibodies recognizing these nuclear proteins. A commercial antibody generated against human protamine-2 (PRM2) has been described to cross-react with mouse PRM2 and in fact has been used in several studies to detect mouse PRM2. Some inconsistent results obtained with this goat-derived antibody prompted us to re-examine its specificity. In immunofluorescence experiments with epididymal sperm, only a low percentage of sperm nuclei were stained by this antibody, whereas a mouse monoclonal anti- PRM2 antibody stained most sperm, as expected. Western blot analysis of basic nuclear proteins from spermatids and sperm separated by acid urea (AU) gel electrophoresis revealed that the goat anti- PRM2 antiserum binds to mouse TNP2 but not mouse PRM2. Epitope mapping using glutathione-S-transferase-fusion proteins with peptide sequences conserved in human PRM2 and mouse TNP2 identified the tetrapeptide arginyl-lysyl-arginyl-threonine as an epitope of the goat anti- PRM2 antiserum. Our findings underline the importance of using AU gel electrophoresis to confirm specificities of antibodies directed against basic nuclear proteins, which are not well separated, and may show abnormal migration behaviour, in SDS-polyacrylamide gel electrophoresis.