Paliyath G, Poovaiah B W
Department of Horticulture and Landscape Architecture, Washington State University, Pullman 99164-6414, USA.
Plant Cell Physiol. 1988;29(1):67-73.
Regulation of the activity of beta-glucan synthase was studied using microsomal preparations from corn coleoptiles. The specific activity as measured by the incorporation of glucose from uridine diphospho-D-[U-14C]glucose varied between 5 to 15 pmol (mg protein)-1 min-1. Calcium promoted beta-glucan synthase activity and the promotion was observed at free calcium concentrations as low as 1 micromole. Kinetic analysis of substrate-velocity curve showed an apparent Km of 1.92 x 10(-4) M for UDPG. Calcium increased the Vmax from 5.88 x 10(-7) mol liter-1 min-1 in the absence of calcium to 9.52 x 10(-7) mol liter-1 min-1 and 1.66 x 10(-6) mol liter-1 min-1 in the presence of 0.5 mM and 1 mM calcium, respectively. The Km values remained the same under these conditions. Addition of ATP further increased the activity above the calcium-promoted level. Sodium fluoride, a phosphoprotein phosphatase inhibitor, promoted glucan synthase activity indicating that phosphorylation and dephosphorylation are involved in the regulation of the enzyme activity. Increasing the concentration of sodium fluoride from 0.25 mM to 10 mM increased glucan synthase activity five-fold over the + calcium + ATP control. Phosphorylation of membrane proteins also showed a similar increase under these conditions. Calmodulin, in the presence of calcium and ATP stimulated glucan synthase activity substantially, indicating that calmodulin could be involved in the calcium-dependent phosphorylation and promotion of beta-glucan synthase activity. The role of calcium in mediating auxin action is discussed.
利用玉米胚芽鞘的微粒体制剂研究了β-葡聚糖合酶活性的调节。通过尿苷二磷酸-D-[U-14C]葡萄糖掺入葡萄糖来测定的比活性在5至15 pmol（mg蛋白质）-1分钟-1之间变化。钙促进β-葡聚糖合酶活性，并且在低至1微摩尔的游离钙浓度下就观察到这种促进作用。底物-速度曲线的动力学分析表明，UDPG的表观Km为1.92×10（-4）M。钙将Vmax从无钙时的5.88×10（-7）mol·升-1·分钟-1分别增加到存在0.5 mM和1 mM钙时的9.52×10（-7）mol·升-1·分钟-1和1.66×10（-6）mol·升-1·分钟-1。在这些条件下，Km值保持不变。添加ATP进一步将活性提高到钙促进水平以上。磷酸蛋白磷酸酶抑制剂氟化钠促进葡聚糖合酶活性，表明磷酸化和去磷酸化参与了酶活性的调节。将氟化钠浓度从0.25 mM增加到10 mM，葡聚糖合酶活性比+钙+ATP对照增加了五倍。在这些条件下，膜蛋白的磷酸化也显示出类似的增加。在存在钙和ATP的情况下，钙调蛋白显著刺激葡聚糖合酶活性，表明钙调蛋白可能参与钙依赖性磷酸化和β-葡聚糖合酶活性的促进。讨论了钙在介导生长素作用中的作用。
