CALMODULIN STIMULATION OF UNIDIRECTIONAL CALCIUM-UPTAKE BY THE ENDOPLASMIC-RETICULUM OF BARLEY ALEURONE

The role of calmodulin (CaM) in gibberellic acid (GA3)-stimulated Ca2+ uptake was investigated in endomembranes isolated from aleurone cells of barley (Hordeum vulgare L.). Unidirectional Ca2+-uptake activity of endoplasmic reticulum (ER) was higher in membranes isolated from aleurone layers treated for 16 h with GA3 and Ca2+ compared with those isolated from layers incubated in Ca2+ alone. However, the level of uptake from Ca2+-treated tissue could be stimulated to that of the GA3-treated cells by applying exogenous CaM which increased the V(max) of the Ca2+ transporter approximately threefold. Calcium uptake in ER from GA3-treated tissue was inhibited by the CaM antagonist W7 in 50% of experiments, whereas the activity in membranes from non-GA3-treated tissue was unaffected. Treatment with GA3 also led to a twofold increase in CaM levels in aleurone layers within 4-6 h, paralleling the time course of the stimulation of Ca2+ uptake and preceding the stimulation of alpha-amylase secretion. We propose that the elevation of Ca2+ uptake into the ER induced by GA3 may be coordinated and regulated by elevated levels of membrane-associated CaM and this may regulate Ca2+-dependent alpha-amylase synthesis in the lumen of the ER.