TAILIEUCHUNG - Báo cáo khoa học: Regulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca2+-sensitivity

In rod phototransduction, cyclic GMP synthesis by mem-brane 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 approxi-mately sums to the cellular concentration of a functional ROS-GC1 dimer. Both GCAPs increase the catalytic effi-ciency (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 | Eur. J. Biochem. 270 3814-3821 2003 FEBS 2003 doi Regulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca2 -sensitivity Ji-Young Hwang1 Christian Lange1 t Andreas Helten1 Doris Hoppner-Heitmann1 Teresa Duda2 Rameshwar K. Sharma2 and Karl-Wilhelm Koch1 1Institut fur Biologische Informationsverarbeitung 1 Forschungszentrum Jiilich Julich Germany 2The Unit of Regulatory and Molecular Biology Departments of Cell Biology and Ophthalmology MIMS SOM UMDNJ Stratford NJ USA In rod phototransduction cyclic GMP synthesis by membrane bound guanylate cyclase ROS-GC1 is under Ca2 -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 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 non-myristoylated GCAP-2 is 10 to 13-fold. GCAPs also confer different Ca2 -sensitivities to ROS-GC1. Activation of the cyclase by GCAP-1 is half-maximal at 707 nM free Ca2 while that by GCAP-2 is at 100 nM. The findings show that differences in catalytic efficiency and Ca2 -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 Ca2 in photoreceptor cells. Keywords phototransduction guanylate cyclase GCAP myristoylation kcat Km. Photoexcitation

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