How We See:  The Visual Cascade

(Under construction.)

          In mammalian systems, light is perceived and transduced by the phototransduction reaction pathway (see Fig. 1). Many forms of acquired or inherited disorders leading to blindness are caused by protein malfunctions within the phototransduction cascade.  This pathway begins when rhodopsin (R), a 7 pass domain membrane-bound protein in rod photoreceptor cells, is activated by light. The activated rhodopsin then binds and activates transducin (T), a G protein composed of three subunits, a, b, and g. The activated, trimeric transducin protein separates into Ta and Tb sections. Ta activates phosphodiesterase, which, in turn, hydrolyzes cGMP. The subsequent decrease of cGMP concentration closes the cGMP-gated channels on the photoreceptor cells, causing a shift in the cell's electric potential and a subsequent neural impulse to the brain.


          Figure 1.  Activation of the phosphodiesterase cascade.  The light-activated hydrolysis of cyclic GMP is shown schematically, with amplification of the photon signal illustrated by circular paths depicted by the arrows.  Light (lightening bolt) absorbed by rhodopsin (R) converts it to an activated state (R*) that then interacts with transducin (GDP a, b, g), initiating GTP-GDP exchange on the alpha subunit.  Next, alpha transducin (GTP-a) dissociates both from R* and beta, gamma transducin (b,g) to bind with an inactive phosphodiesterase (PDE) enzyme, stimulating it (PDE*) to hydrolyze cyclic GMP to 5'GMP.  Hydrolysis of GTP by intrinsic GTPase activity of alpha transducin releases GDP-a from the PDE allowing recombination with its beta, gamma subunits.  (Original figure created by Lolley, R.N, Craft, C.M., and Lee, R.H., 1991.  Modified by Carlson, B.L.)