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

Total actin and its filamentous (F) and globular (G) forms were quantified in the wheat (Triticum aestivum L.) callus culture. The total amount 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-actin correlated with the proportion of morphogenic callus tissue, where it comprised up to 50% of total actin. In the soft, hydrated callus, the content of F-actin decreased to nearly zero. A fraction of heat-stable actin-binding proteins was isolated from the wheat callus. The polypeptides with mol wts of 30 (a major polypeptide), 35.5, and 38 kD were identified as tropomyosin isoforms on the basis of their thermostability, isoelectric point (4.5), and ability for a Mg2+-dependent actin binding. The heat-stable F-actin-binding polypeptides with apparent mol wts of 86 and 83 kD appear to be the light caldesmon isoforms. The suggestion was made that the cell potential for differentiation was related to the state of the actin cytoskeleton. Further methods for tropomyosin and caldesmon identification in plant tissues as well as the occurrence of specific isoforms of these proteins in callus cultures are discussed.