Changes in Protein Prenylation and Prenyltransferase Activity in the Rat Seminiferous Epithelium during Early Stages of Spermatogenesis1

Abstract

Changes in protein prenyltransferase activity, levels of prenylated protein, and the type of isoprenoid modification was described in cells of rat seminifereous epithelium and correlated with differentiative events of spermatogenesis. The activity of protein farnesyltransferase (PFT) was at least 10-fold higher than that for protein geranylgeranyltransferase-I (PGGT-I) in seminiferous epithelium and spermatogenic cells of prepubertal rats of different ages. Both activities increased during the meiotic stages of differentiation and peaked at 23 days of age. The activity of farnesyltransferase in seminiferous epithelium was the same as that in mixed spermatogenic cell populations from animals aged 9 and 23 days, indicating that the activity of this enzyme in somatic cells and germ cells was similar at these ages. Farnesyltransferase activities were similar and low in both pachytene spermatocytes and round spermatids from adult rats; however, the activity in pachytene spermatocytes from 23-day old animals was 2-fold higher than in adults. The highest activity was associated with intermediate-sized spermatocytes appearing late during meiosis. PGGT-I activity was at least 10-fold lower than farnesyltransferase activity and was not significantly different among all cell populations. Differentiation-dependent in vivo protein prenylation was demonstrated by labeling of seminiferous epithelium with [3H]mevalonic acid at different prepubertal ages. Total protein prenylation and the ratio of geranylgeranylated to farnesylated protein, in contrast to prenyltransferase activity, decreased with increasing age. Although 20-30-kDa proteins were the most highly labeled at all ages, [3H]-proteins from different-aged prepubertal rats showed age-dependent changes in the level of prenylation of at least 14 proteins as determined by two-dimensional (2D) electrophoresis. Prenylated proteins of round spermatids were distinguished from those of the spermatocytes by the lack of many 20-30-kDa proteins and by low geranylgeranyl/farnesyl (GG/F) ratios. These results show that independent changes in prenyltransferase activity and protein prenylation accompany the differentiation events during the premeiotic and meiotic stages of spermatogenesis. This suggests that prenylation in the seminiferous epithelium may be more dependent on available protein substrate than on protein prenyltransferase activity.