Stimulation of Ca 2+-pumping ATPase activity in carrot plasma membrane by calmodulin

Abstract

Activity of Ca 2+-pumping ATPase located at the plasma membrane of cultured carrot cells was appreciably stimulated when the enzyme was associated with calmodulin (CAM). Affinity of the ATPase toward the substrates was increased by the addition of CAM, and K m s of the enzyme for Ca 2+ were estimated to be 11.4 and 0.7 μM in the absence and presence of CAM, respectively, while the values for ATP decreased only slightly (914 and 670 μM). In contrast, relative V values of the enzyme were essentially not changed even after the addition of CAM. This ATPase was capable of utilizing several ATP analogues, such as GTP, ITP, UTP and CTP, instead of ATP, and Ca 2+-transport driven by these alternative substrates was also stimulated by CAM. The molar ratio of Ca 2+-transport and ITP hydrolysis of the enzyme reaction was estimated to be 2:1. These observations suggested that CAM-induced stimulation of Ca 2+- ATPase activity at the plasma membrane of cultured carrot cells is mainly due to the increase in the affinity of the enzyme toward Ca 2+ by association with the modulator protein, and the hydrolysis of 1 mol of ATP by the enzyme action results in the transport of 2 mol of Ca 2+ across the membrane. © 1997 Elsevier Science Ltd. All rights reserved