Promotion of beta-glucan synthase activity in corn microsomal membranes by calcium and protein phosphorylation.
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.
Paliyath
G
G
Department of Horticulture and Landscape Architecture, Washington State University, Pullman 99164-6414, USA.
Poovaiah
B W
BW
D000255:Adenosine Triphosphate / Q000502:physiology; D029681:Arabidopsis Proteins; D002118:Calcium / Q000502:physiology*; D002147:Calmodulin / Q000502:physiology*; D018548:Cotyledon / Q000166:cytology / Q000201:enzymology / Q000254:growth & development; D005964:Glucosyltransferases / Q000378:metabolism*; D007210:Indoleacetic Acids / Q000502:physiology; D008565:Membrane Proteins; D008861:Microsomes / Q000378:metabolism; D010766:Phosphorylation; D029702:Schizosaccharomyces pombe Proteins; D015290:Second Messenger Systems / Q000502:physiology*; D012969:Sodium Fluoride / Q000378:metabolism; D003313:Zea mays / Q000166:cytology / Q000201:enzymology* / Q000254:growth & development
D029681:Arabidopsis Proteins; D002147:Calmodulin; D007210:Indoleacetic Acids; D008565:Membrane Proteins; D029702:Schizosaccharomyces pombe Proteins; D000255:Adenosine Triphosphate; D012969:Sodium Fluoride; D005964:Glucosyltransferases; C457769:PRC1 protein, Arabidopsis; C028634:cellulose synthase (UDP-forming); C020570:1,3-beta-glucan synthase; D002118:Calcium