Molecular Biology Reports 14: 261-264, 1990. 9 1990 Kluwer Academic Publishers. Printed in Belgium. 261 Effect of post-synthetic modifications of proteins on the binding of estrogen-receptor complex to uterine nuclei of aging rats J. Kaur & M. K. Thakur* Biochemistry & Molecular Biology Laboratory, Department of Zoology, Banaras Hindu University, Varanasi 221 005, India (* requests for offprints) Received 4 May 1990; accepted in revised form 23 July 1990 Key words: Post-synthetic modifications, estrogen receptor, chromatin, aging, (rat uterus) Abstract The binding of estrogen-receptor (ER) complex to nuclei following post-synthetic modifications of proteins was examined in the uteri of young (18 weeks) and old (96 weeks) rats. Acetylation decreases the binding of ER complex to nuclei but methylation shows no effect on the extent of binding in both ages. On the other hand, phosphorylation enhances the binding of ER complex by two-fold in nuclei from young rats but reduces this to half in nuclei from old rats. The pattern of binding in salt-resistant nuclear fractions is similar to that in total nuclei except in methylation where old rats show about 20 ~o higher binding as compared to the respective control. These findings suggest that post-synthetic modifications of proteins modulate the binding of ER complex to uterine nuclei in an age-specific manner. Introduction Estrogen binds to its cognate intracellular recep- tor and the resulting estrogen-receptor (ER) com- plex undergoes activation and then interacts with the acceptor site on chromatin. This alters the conformation of chromatin and expression of specific genes. Such alteration modulates the level of gene products and thereby controls various cellular functions [1-4]. We have previously reported that chromoso- mal proteins undergo age-dependent post-syn- thetic modifications by acetylation, phosphoryla- tion and methylation [5, 6]. Furthermore, it has been demonstrated that phosphorylation of chromatin enhances its affinity for binding to androgen receptors in target tissues of young rats [7]. Also the covalent modification of ER by phosphorylation has been well documented [8-10]. It influences the binding of estrogen to receptor [ 11 ] as well as the binding of ER complex to chromatin [8, 12]. Thus it is likely that both the modifications of receptors as welt as of chromo- somal proteins influence the binding of ER com- plex to nuclei. Keeping the above facts in mind, we have examined the effect of in vitro post-synthetic modifications of proteins such as acetylation, phosphorylation and methylation on the binding of ER complex to uterine nuclei of young and old rats.