The PI Cascade Generates at Least Two Intracellular Messenger Molecules

The PI cascade generates two messenger molecules with very different properties (Figure 5) DAG is essentially confined to the plasma membrane, while IP3 can diffuse into the surrounding cytoplasm. In addition, the decline of the precursor, PIP2 may act as an additional signal within


Figure 5 Diagram of mechanisms proposed to mediate phototransduction within a microvillus of the Limulus photoreceptor. cGMP, cyclic guanosine monophosphate; DAG, diacylglycerol; ER, endoplasmic reticulum; Gqa, alphasubunit of a heterotrimeric GTP-binding protein of the Gq subfamily; p, y, beta and gamma subunits of a heterotrimeric GTP binding protein of the Gq sub-family; GC, guanylate cyclase; IP3, d-myo-inositol (1,4,5) trisphosphate; IP3R, d-myo-inositol (1,4,5) trisphosphate receptor; PLC, phospholipase C; Rh, rhodopsin; TRPC, transient receptor channel C.

The membrane. The difficult task of assessing the relative roles of these messengers in activating the light-sensitive ion channels has dominated research on the invertebrate phototransduction cascade.

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