Re-entry in cell cycle: protein metabolism and transglutaminase-like activity in Helianthus tuberosus

Slices of Helianthus tuberosus L. cv OB1 tuber were activated to induce a synchronous cell cycle, during various phases of tuber dormancy. The reactivity expressed as protein content and the TGase activity are highest during deep dormancy and low during tuber formation and dormancy release. At the onset of the cycle an abrupt decrease in total protein and synthesis of several leucine- and methionine-labelled proteins occur. The solubility of the proteins also changes. After mid-G1, synthesis prevails and thus protein levels increase. The synthesis of the components of a temperature-sensitive (Ts) and -insensitive (Ti) transglutaminase-like (TGase-like) activity is cycloheximide inhibited and begins before mid-G1, but their activity is evident only later on. While the products of the Ts activity, are polyamine-protein conjugates of high molecular mass, the products of the Ti activity are not covalently bound. A possible role of endogenous proteases in this binding activity is discussed.