Fig. 4. Schematic model illustrating the activity–dependence of modulation of cyclic-nucleotide-gated (CNG) channels by tyrosine phosphorylation and dephosphorylation. The top and bottom diagrams represent closed and open channels, respectively; the left and right diagrams represent phosphorylated and dephosphorylated channels, respectively. The ovals represent cGMP molecules. The relative thickness of the arrowheads represents changes in the favorability of gating. PTK, protein tyrosine kinase; PTP, protein tyrosine phosphatase.