Figure 5.

A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (G_q). G_q-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP₂) producing InsP₃. InsP₃ opens Ca²⁺ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca²⁺ into the cytosol. Ca²⁺ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.