Protein phosphorylation in developing and regenerating rat kidney

Abstract

Renal cytosolic extracts from rats of different ages and mononephrectomized rats were incubated with Y- [~~P]ATP and analysed by high resolution two-dimensional electrophoresis and autoradiography. Extracts from new-born and young rats showed a great number of phosphorylated proteins migrating between the origin and Mr 52,000. Among these proteins, the group co-migrating with phosphorylase b (Mr 97,000) was particularly evident in new-born and days-old rats. In extracts from mature rats, other proteins of lower molecular weight, particularly those migrating between Mr 60,000 and 44,000, became intensely phosphorylated. The number and intensity of phosphorylated proteins from extracts of normal and nephrectomized rats, however, did not vary. Activity of CAMP-dependent protein kinase and [3H]cAMP binding was also modified during neonatal development but not in compensatory renal growth. Since CAMP-PK and protein phosphorylation are known to be regulated in response to hormonal stimulations, these results may provide good indications for the understanding of hormonal involvement in kidney growth.Protein phosphorylation has been recognized to be the major general mechanism by which intracellular events occur in response to neural and hormonal stimuli (Cohen, 1982). A large number of polypeptide hormones and some non-peptide agents (including catecholamines, isoproterenol and thyroxine) exert their effects by increasing or decreasing the concentration of cyclic AMP (CAMP). The effects of this second messenger on metabolic pathways are then mediated by CAMP-dependent protein kinase (CAMP-PK) through the phosphorylation of various regulatory enzymes (Cohen, 1988).The intracellular receptor for cAMP that has been identified in tissues is the regulatory subunit of CAMP-PK. Binding of cAMP to the regulatory subunit of the protein kinase holoenzyme causes a dissociation of the enzyme, freeing the catalytic subunit which catalyses the incorporation of phosphate into specific serine and threonine residues, using the terminal phosphate group of ATP as the phosphate donor (Brostom et al., 1970). Cyclic AMP has been demonstrated to mediate actions of several hormones that have an effect on renal cells