ACTIN AND HEAT-STABLE ACTIN-BINDING PROTEINS IN WHEAT CALLUS-CULTURE

Total actin and its filamentous (F) and globular (G) forms were quanti fied in the wheat (Triticum aestivum L.) callus culture. The total amo unt of actin remained essentially constant (ca. 9 mu g/mg of extracted protein) during the long-term callus culturing, and did not differ in the morphogenic and nonmorphogenic callus zones. The amount of F-acti n correlated with the proportion of morphogenic callus tissue, where i t comprised up to 50% of total actin. In the soft, hydrated callus, th e content of F-actin decreased to nearly zero. A fraction of heat-stab le actin-binding proteins was isolated from the wheat callus. The poly peptides with mol wts of 30 (a major polypeptide), 35.5, and 38 kD wer e identified as tropomyosin isoforms on the basis of their thermostabi lity, isoelectric point (4.5), and ability for a Mg2+-dependent actin binding. The heat-stable F-actin-binding polypeptides with apparent mo l wts of 86 and 83 kD appear to be the light caldesmon isoforms. The s uggestion was made that the cell potential for differentiation was rel ated to the state of the actin cytoskeleton. Further methods for tropo myosin and caldesmon identification in plant tissues as well as the oc currence of specific isoforms of these proteins in callus cultures are discussed.