TMEM16A-TMEM16B chimaeras to investigate the structure-function relationship of calcium-activated chloride channels

TMEM16A and TMEM16B proteins are CaCCs (Ca2+-activated Cl- channels) with eight putative transmembrane segments. As shown previously, expression of TMEM16B generates CaCCs characterized by a 10-fold lower Ca2+ affinity and by faster activation and deactivation kinetics with respect to TMEM16A. To investigate the basis of the different properties, we generated chimaeric proteins in which different domains of the TMEM16A protein were replaced by the equivalent domains of TMEM16B. Replacement of the N-terminus, TMD (transmembrane domain) 1-2, the first intracellular loop and TMD3-4 did not change the channel's properties. Instead, replacement of intracellular loop 3 decreased the apparent Ca2+ affinity by nearly 8-fold with respect to wild-type TMEM16A. In contrast, the membrane currents derived from chimaeras containing TMD7-8 or the C-terminus of TMEM16B showed higher activation and deactivation rates without a change in Ca2+ sensitivity. Significantly accelerated kinetics were also found when the entire C-terminus of the TMEM16A protein (77 amino acid residues) was deleted. Our findings indicate that the third intracellular loop of TMEM16A and TMEM16B is the site involved in Ca2+-sensitivity, whereas the C-terminal part, including TMD7-8, affect the rate of transition between the open and the closed state.