Biologjia Online

An activation switch in the rhodopsin family of G protein coupled

We aimed at understanding molecular events involved in the activation of a member of the G protein coupled receptor family, the thyrotropin receptor. We have focused on the transmembrane region and in particular on a network of polar interactions between highly conserved residues. Using molecular dynamics simulations and site-directed mutagenesis techniques we have identified residue N7.49, of the NPxxY motif of TM 7, as a molecular switch in the mechanism of TSHr activation. N7.49 appears to adopt two different conformations in the inactive and active states. These two states are characterized by specific interactions between this Asn and polar residues in the transmembrane domain. The inactive gauche+ conformation is maintained by interactions with residues T6.43 and D6.44. Mutation of these residues into Ala increases the constitutive activity of the receptor by factors of ~14 and ~10 relative to wt TSHr, respectively. Upon receptor activation N7.49 adopts the trans conformation to interact with D2.50 and a putatively charged residue that remains to be identified, as shown by the fact that D2.50A, D2.50N, D2.50N/N7.49D, and N7.49D mutants can all be activated by TSH. In addition, the conserved L2.46 of the (N/S)LxxxD motif also plays a significant role in restraining the receptor in the inactive state since the L2.46A mutation increases constitutive activity by a factor of ~13 relative to wt TSHr. As residues L2.46, D2.50, and N7.49 are strongly conserved, this molecular mechanism of TSHr activation can be extended to other members of the rhodopsin-like family of G protein-coupled receptors.

posted by Selfmaderadio @ 12:06 e pasdites