[Age-dependent changes in the phosphorylation of nuclear phosphoproteins of rat salivary glands]

Development, growth, maturation and aging processes of secretory cells of rat salivary glands progress mainly after birth. Nuclear non-histone proteins, phosphorylated actively and reversively, have an important role as regulatory molecules of gene activity and have a possibility to bring about specific changes in these cellular processes. We examined in the present study the age-dependent changes in the phosphorylation of non-histone proteins of rat salivary glands. Nuclei purified from submandibular and parotid glands of 8-week-old rats rapidly incorporated 32P from gamma-32P-ATP into the nuclear phosphoproteins and reached equilibrium within 9 min. A preponderant amount of the 32P was present in non-histone proteins. The levels of phosphorylation of non-histone proteins in salivary gland nuclei increased rapidly after birth, reaching a maximum in both gland nuclei of 4-week-old rats and then decreasing to the levels observed in submandibular and parotid gland nuclei from 20 and 16-week-old rats, respectively. These levels were still maintained in nuclei from aged rats. Moreover, age-dependent changes in the protein kinase activity of submandibular and parotid gland nuclei were linked up with the changes in the phosphorylation of non-histone proteins. However, changes were not observed in the phosphorylation of histone proteins after birth. These results suggest that protein kinase activity in salivary gland nuclei may have an important role on age-dependent changes in cell function, mediated through the control of the phosphorylation of non-histone proteins.