Ultrasensitive signal detection by a guanylyl cyclase chemoreceptor

Abstract

Background Sperm navigate to the egg in a gradient of a chemoattractant for fertilization – a mechanism called chemotaxis. In sea urchin, the chemoattractant peptide binds to a chemoreceptor guanylyl cyclase (GC) on the sperm surface. Activation of the GC initiates a sequence of signaling events that eventually results in Ca2+ influx and a change in swimming direction. We studied the GC properties that allow sperm to track the chemoattractant with singlemolecule precision on a millisecond time scale. A high density (9•103 GC molecules/μm2) and a subnanomolar ligand affinity provide a high ligand-capture efficacy. The sperm surface represents an almost perfect absorber. The peptide-induced GC activity is terminated by multiple dephosphorylation steps, which provide a means of precise lifetime control and, thereby, reduces “molecular noise”. Several experiments suggest that GC undergoes autodephosphorylation. Future experiments need to clarify, whether the GC entertains phosphatase and kinase activity, possibly in the kinase-homology domain (KHD).