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

Abstract

The ε-(γ-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 Mg 2+-ATP and Mg 2+-ATP plus Ca 2+. 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 Mg 2+-ATP and an increase in cross-link content upon addition of Mg 2+-ATP plus Ca 2+. In cultured embryonic chick skeletal myofibrils, we have observed a decrease in cross-link content upon addition of Mg 2+-ATP. In cultured embryonic chick heart myofibrils, we have observed an increase in cross-link content upon addition of Mg 2+-ATP followed by Ca 2+. 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 crosslinks. In such a hypothesis, the role of Mg 2+-ATP is that of introducing energy which can be used for the cycling of cross-links and possibly for some cellular energy transductions.