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 prot ein that was originally discovered as a member of steroid receptor com plexes. FKBP59 is ubiquitous and well conserved, and it appears to be a multifunctional protein. It has peptidylprolyl cis-trans-isomerase a ctivity and therefore may be involved in protein folding as a molecula r chaperon. FKBP59 also includes in its structure a tetratricopeptide repeat (TPR) motif and could have a function in the cell division proc ess. In situ hybridization experiments revealed an overexpression of F KBP59 mRNA in rabbit and rat testes in comparison with other organs. T his high level of expression was restricted to germ cells of the semin iferous epithelium. Increasing levels of FKBP59 mRNA became obvious fr om the midpachytene stage, and the strongest signal was observed in th e late pachytene, diplotene, and diakinesis primary spermatocytes. The expression then declined progressively in postmeiotic early spermatid s. High expression of FKBP59 mRNA did not occur in earlier and later g erm cell stages. During prepubertal development, Northern blot and in situ hybridization of rat testes examined at various postnatal ages re vealed that FKBP59 mRNA was not expressed at over a basal level until the pachytene stage. High expression of the FKBP59 protein was demonst rated in the rabbit testis by Western blot and was localized by immuno histochemistry 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 sugges ts that FKBP59 is involved in a specific function during the cell divi sion process.