Proteolytic activation and stimulation by Ca²⁺ of glucan synthase from soybean cells

Abstract

In homogenates from suspension‐cultured soybean cells, 1,3‐β‐d‐glucan synthase activity is increased through preincubation with trypsin or due to action of an endogenous enzyme which presumably is a protease as it is inhibited by soybean trypsin inhibitor. The 1,3‐β‐d‐glucan synthase in untreated membrane preparations is also reversibly stimulated by Ca2+. This Ca2+‐dependence is lost on proteolytic activation. Regardless as to whether the enzyme was rendered active by preincubation with trypsin or by the presence of Ca2+, it is inhibited by calmidazolium, trifluoperazine and polymyxin B, suggesting that the activation by Ca2+ is not mediated by calmodulin.

Chitosan-Elicited Callose Synthesis in Soybean Cells as a Ca2+-Dependent Process

“…For more details see "Discussion". activation taking place in crude homogenates (17). However, the present observation that extracts of chitosan-treated cells do not show an increase in Ca2"-independent 1,3-,B-glucan synthase activity (Table III) shows that this proteolytic step has no physiological significance during chitosan-induced callose synthesis.…”

“…In the present report we present some quantitative and regulatory aspects of callose formation which might be understood taking into account the recently observed Ca2"-requirement of the l,3-fl-glucan synthase (17). The results also allow some hypothetical suggestions toward a possible role of Ca2" in the induction of other chitosan-induced physiological changes regarded to be of importance in resistance phenomena.…”

“…When crude cell homogenates, resuspended 480g-pellet (= membranes adhering to cell walls) or washed microsomal particles (17) were used for experiments similar to those reported in Table III, no significant differences between control and chitosan-treated cells were found.…”

“…The homogenate was centrifuged for 3 min at 480g and the supernatant used in the enzyme assay. Alternatively, a microsomal fraction was prepared from this supernatant as described (17) (17).…”

“…These values were determined using a Ca2+-sensitive electrode (17) and a Ca2+-standard diluted in quartz-distilled H20 containing 5 mm MgCl2. A concentration of free Ca2' around 100 Mm is saturating when the 1,3-,B-glucan synthase is assayed under the above conditions (17).…”