A switch in the phosphorylation state of the dimeric form of the Meg1 protein correlates with progression through meiosis in the mouse

meg1 is a murine gene that encodes for a 0.75-kb transcript that in mature male mice is expressed exclusively in the testis. This transcript starts to accumulate in early stages of the first meiotic prophase and reaches a peak in pachytene spermatocytes. In females, meg1 transcripts are detectable only in ovaries of embryos with oocytes that have reached the prophase stage of the first meiotic division. No meg1 transcripts can be detected in adult ovaries. meg1 is, therefore, assumed to be involved with meiotic processes. In this study, specific polyclonal antibodies were raised against the Meg1 protein and were used to demonstrate that this protein is indeed specific to the testis. Western blot analysis of immunoprecipitated Meg1 protein revealed multiple bands (in the range of M(r) 12,000-18,000), some of which where recognized by anti-phosphotyrosine antibodies, suggesting that in vivo, Meg1 appears in multiple phosphorylated forms. Western analysis of purified M(r) 15,000 recombinant Meg1 protein, under nonreducing conditions, revealed an apparent M(r) 31,000 band, suggesting that Meg1 can form a homodimer via S-S bonds. Analysis of Meg1 from postnatal testes at different developmental stages revealed that in addition to the multiple monomeric forms of Meg1, two dimeric forms of about M(r) 31,000 and M(r) 32,000 were consistently detected. A developmentally regulated switch in the relative predominance of these two dimeric forms was apparent. The M(r) 31,000 form, which is tyrosine phosphorylated, becomes the predominant form once the cells enter meiosis. These results suggest that dimerization and phosphorylation/dephosphorylation reactions might regulate the function of Meg1 during meiosis.