Light cascade

Photoactivated rhodopsin triggers a series of
enzymatic steps (light cascade). First, a signaltransmitting
protein of visualization, transducin,
is activated by photoactivated rhodopsin.
Transducin belongs to the G protein family, i.e.,
it can assume an inactive GDP and an active GTP
form. GTP activates phosphodiesterase. This
very rapidly hydrolyzes cGMP and lowers the
cGMP concentration in cytosol, which leads to
closure of the sodium ion channels. Immediately
thereafter, phosphodiesterase is inactivated
by means of a G protein cycle.