Stage-Specific Expression of the Immunophilin FKBP59 Messenger Ribonucleic Acid and Protein during Differentiation of Male Germ Cells in Rabbits and Rats

FKBP59 is an immunophilin that binds the immunosuppressant drugs FK506 and rapamycin. It is a 90-kDa heat shock protein (hsp90)-binding protein that was originally discovered as a member of steroid receptor complexes. FKBP59 is ubiquitous and well conserved, and it appears to be a multifunctional protein. It has peptidylprolyl cis-trans-isomerase activity and therefore may be involved in protein folding as a molecular chaperon. FKBP59 also includes in its structure a tetratricopeptide repeat (TPR) motif and could have a function in the cell division process. In situ hybridization experiments revealed an overexpression of FKBP59 mRNA in rabbit and rat testes in comparison with other organs. This high level of expression was restricted to germ cells of the seminiferous epithelium. Increasing levels of FKBP59 mRNA became obvious from the midpachytene stage, and the strongest signal was observed in the late pachytene, diplotene, and diakinesis primary spermatocytes. The expression then declined progressively in postmeiotic early spermatids. High expression of FKBP59 mRNA did not occur in earlier and later germ cell stages. During prepubertal development, Northern blot and in situ hybridization of rat testes examined at various postnatal ages revealed that FKBP59 mRNA was not expressed at over a basal level until the pachytene stage. High expression of the FKBP59 protein was demonstrated in the rabbit testis by Western blot and was localized by immunohistochemistry from late pachytene spermatocytes to round spermatids. The cell type-specific and developmental stage-specific expression of FKBP59 at a restricted period of male germ cell differentiation suggests that FKBP59 is involved in a specific function during the cell division process.