Open AccessRegulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca 2+ ‐sensitivity
Author(s) -
Hwang JiYoung,
Lange Christian,
Helten Andreas,
HöppnerHeitmann Doris,
Duda Teresa,
Sharma Rameshwar K.,
Koch KarlWilhelm
Publication year - 2003
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1046/j.1432-1033.2003.03770.x
Subject(s) - sensitivity (control systems) , chemistry , biophysics , catalysis , membrane , guanylate cyclase , physics , biology , biochemistry , enzyme , electronic engineering , engineering
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 ( k cat / K m ) 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 k cat / K m . 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 n m free [Ca 2+ ], while that by GCAP‐2 is at 100 n m . 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.