A Receptor Guanylyl Cyclase Reveals Auto-Phosphatase Activity

Abstract

In sea urchin sperm of Arbacia punctulata, a guanylyl cyclase (GC) serves as chemotaxis receptor that enables sperm to respond to a single molecule of the chemoattractant, resact. The efficiency of resact capture is high, because GC covers about 50% of the flagellar surface and binds resact with picomolar affinity. Furthermore, the binding affinity is controlled by the level of occupancy of the receptor. At high occupancy the resact affinity is lowered through negative cooperativity among subunits of the trimeric GC complex. The lifetime of active GC is controlled by its phosphorylation state. At rest GC is phosphorylated at six serine residues. After activation by resact, the GC becomes dephosphorylated with a biphasic time course, whereas dephosphorylation strongly coincides with the decrease of cGMP synthesis. During the initial fast phase the amplitude of GC dephosphorylation increases with the occupancy level. However, the time constant of this phase is independent of receptor occupancy. Moreover, dephosphorylation is superstoichiometric: even if only 5% of the GCs are occupied by resact, approximately 70% become dephosphorylated. We conclude from these results, that the occupied GC inactivates by auto-dephosporylation (fast phase) and additionally can dephosphorylate adjacent non-occupied GCs (slow phase). We could show for the first time that a receptor GC is regulated by auto-dephosphorylation.