Figure 5
CNG channel subunit composition alters channel gating kinetics. (A) CNG subunits were expressed heterologously in human embryonic kidney (HEK) cells. The physiology of HEK cells triply transfected with the CNG channel subunit TAX-2, TAX-4, and CNG-3 was examined and found to exhibit a similar profile to the double transfection of TAX-4 with TAX-2. The K½ for TAX-2/TAX-4/CNG-3 transfected cells was 2.72 µM. (B) Next, the CNG channel subunits TAX-4 and TAX-2 were transfected into HEK cells, and cells expressing both subunits were studied using patch recordings after cGMP application. The double transfected cells exhibit fast opening and closing gating kinetics. (C,D) The physiology of HEK cells transfected with TAX-2 and TAX-4 (C) were compared with cells that were transfected with TAX-4 and a mutant allele of TAX-2 in which the candidate PKG which was previously shown to be necessary for short-term adaptation is mutated to an Alanine (D). The half activation concentration (K½) is reduced from 3.53 µM to 1.39 µM of cGMP. (E) TAX-4 exhibits a very slow closing kinetics on account of high cGMP affinity. (F) A model of the data is presented that suggests a native channel conformation consisting of CNG-3, TAX-2, and TAX-4. TAX-2 has previously been shown to contain a candidate PKG site important for short-term adaptation19 and here we reveal a candidate PKG site in CNG-3 whose phosphorylation state tunes odor sensitivity in AWC. CNG-3 and TAX-2 may represent targets of EGL-4 at possibly different time scales of the adaptation.
