Modulation of the epsilon-(gamma-glutamic)lysine cross-link in cellular proteins. I. In vivo and in vitro studies

The epsilon-(gamma-glutamic)lysine cross-link content of intracellular proteins in a variety of cell types can be modulated in vivo by temperature changes and in vitro by treatment with Mg2+/-ATP and Mg2+/-ATP plus Ca2+. Virtually all the cross-links are found in the cytoskeletal and membrane components which are not solubilized by glycerol extraction. In the slime mold, Physarum polycephalum, enzyme activities persist which bring about a decrease in cross-link content upon addition of Mg2+/-ATP and an increase in cross-link content upon addition of Mg2+/-ATP plus Ca2+. In cultured embryonic chick skeletal myofibrils, we have observed a decrease in cross-link content upon addition of Mg2+/-ATP. In cultured embryonic chick heart myofibrils, we have observed an increase in cross-link content upon addition of Mg2+/-ATP followed by Ca2+. A hypothesis is discussed in which the modulation of Glu-Lys cross-links is explained in terms of a cycle of reactions which involves (1) the formation of an acyl phosphate of a glutamic acid side chain; (2) the formation of a Glu-Lys cross-link, and (3) the hydrolytic or phosphorylitic breakdown of the cross-links. In such a hypothesis, the role of Mg2+/-ATP is that of introducing energy which can be used for the cycling of cross-links and possibly for some cellular energy transductions.