title: Regulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca(2+)-sensitivity.
abstract: In rod phototransduction, cyclic GMP synthesis by membrane bound guanylate cyclase ROS-GC1 is under Ca(2+)-dependent negative feedback control mediated by guanylate cyclase-activating proteins, GCAP-1 and GCAP-2. The cellular concentration of GCAP-1 and GCAP-2 approximately sums to the cellular concentration of a functional ROS-GC1 dimer. Both GCAPs increase the catalytic efficiency (kcat/Km) of ROS-GC1. However, the presence of a myristoyl group in GCAP-1 has a strong impact on the regulation of ROS-GC1, this is in contrast to GCAP-2. Catalytic efficiency of ROS-GC1 increases 25-fold when it is reconstituted with myristoylated GCAP-1, but only by a factor of 3.4 with nonmyristoylated GCAP-1. In contrast to GCAP1, myristoylation of GCAP-2 has only a minor effect on kcat/Km. The increase with both myristoylated and nonmyristoylated GCAP-2 is 10 to 13-fold. GCAPs also confer different Ca(2+)-sensitivities to ROS-GC1. Activation of the cyclase by GCAP-1 is half-maximal at 707 nM free [Ca(2+)], while that by GCAP-2 is at 100 nM. The findings show that differences in catalytic efficiency and Ca(2+)-sensitivity of ROS-GC1 are conferred by GCAP-1 and GCAP-2. The results further indicate the concerted operation of two 'GCAP modes' that would extend the dynamic range of cyclase regulation within the physiological range of free cytoplasmic Ca(2+) in photoreceptor cells.
authors.affiliation: Institut für Biologische Informationsverarbeitung 1, Forschungszentrum Jülich, Jülich, Germany.
authors.forename: Ji-Young
authors.initials: JY
authors.lastname: Hwang
authors.forename: Christian
authors.initials: C
authors.lastname: Lange
authors.forename: Andreas
authors.initials: A
authors.lastname: Helten
authors.forename: Doris
authors.initials: D
authors.lastname: Höppner-Heitmann
authors.forename: Teresa
authors.initials: T
authors.lastname: Duda
authors.forename: Rameshwar K
authors.initials: RK
authors.lastname: Sharma
authors.forename: Karl-Wilhelm
authors.initials: KW
authors.lastname: Koch
mesh_terms: D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D002384:Catalysis; D002417:Cattle; D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects; D004926:Escherichia coli / Q000378:metabolism; D006162:Guanylate Cyclase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D019814:Myristic Acid / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D012374:Rod Cell Outer Segment / Q000201:enzymology*; D014162:Transfection
chemical_list: D002135:Calcium-Binding Proteins; D011994:Recombinant Proteins; D019814:Myristic Acid; D006162:Guanylate Cyclase; D002118:Calcium