Ca²⁺ sensor S100β-modulated sites of membrane guanylate cyclase in the photoreceptor-bipolar synapse

This study documents the identity of a calcium-regulated membrane guanylate cyclase transduction system in the photoreceptor-bipolar synaptic region. The guanylate cyclase is the previously characterized ROS-GC1 from the rod outer segments and its modulator is S100β. S100β senses increments in free Ca²⁺ and stimulates the cyclase. Specificity of photoreceptor guanylate cyclase activation by S100β is validated by the identification of two S100β-regulatory sites. A combination of peptide competition, surface plasmon resonance binding and deletion mutation studies has been used to show that these sites are specific for S100β and not for another regulator of ROS-GC1, guanylate cyclase-activating protein 1. One site comprises amino acids (aa) Gly962-Asn981, the other, aa Ile1030-Gln1041. The former represents the binding site. The latter binds S100β only marginally, yet it is critical for control of maximal cyclase activity. The findings provide evidence for a new cyclic GMP transduction system in synaptic layers and thereby extend existing concepts of how a membrane-bound guanylate cyclase is regulated by small Ca²⁺-sensor proteins.