Changing activities and different resistance to proteolytic activity of two forms of glutamine synthetase in wheat leaves during senescence

Abstract

Leaves of field-grown wheat (Triticum aestivum) were collected at different stages. Two forms of glutamine synthetase (GS I and GS II) were separated on diethylamino ethyl cellulose columns with a linear NaCl gradient. GS II was the predominant form in young leaves (85-92% of total activity). The activity of GS II decreased earlier than that of GS I during senescence, and therefore the ratio GS I/GS II changed in favour of GS I. Total glutamine synthetase was inactivated more rapidly in senescing leaves (high caseolytic activity) than in extracts of young leaves (low caseolytic activity). A more rapid inactivation of GS II than of GS I was found when extracts were pre-incubated at 30°C and then applied to a diethylamino ethyl cellulose column, suggesting that GS I was less susceptible to proteolysis than GS II. GS II is assumed to be located in the chloroplasts and GS I in the cytoplasm. Besides subcellular localization, the different stability may contribute to the changing ratio GS I/GS II.