Effect of inactivating mutations on phosphorylation and internalization of the human VPAC2 receptor

The VPAC(2) receptor, as all members of the G-protein-coupled receptor (GPCR)-B family, has two highly conserved motifs in the third intracellular (IC(3)) loop: a lysine and a leucine located at the amino-terminus and two basic residues separated by a leucine and an alanine at the carboxyl-terminus. This study evaluates the involvement of those conserved amino acid sequences in VPAC(2) signal transduction and regulation. The residues were mutated into alanine and mutants were expressed in Chinese hamster ovary (CHO) cells stably transfected with Galpha16 and aequorin. Mutation of L310 reduced efficacy of vasoactive intestinal polypeptide (VIP) to stimulate adenylate cyclase activity through Galphas coupling by 75%, without affecting VIP capability to stimulate an increase in [Ca(2+)](i) through Galpha16 coupling. Mutation of R325 and, to a lesser extend, K328 reduced VIP efficacy to stimulate [Ca(2+)](i) increase and VIP potency to stimulate adenylate cyclase. The combination of mutations of both amino- and carboxyl-terminus located conserved motifs of the IC(3) loop generates an inactive receptor with respect to [Ca(2+)](i) increase and adenylate cyclase activation, but also with respect to receptor phosphorylation and internalization that were indeed directly correlated with the potency of inactivation of the receptors. The amino-terminus of the VPAC(2) receptor IC(3) loop is thus involved in adenylate cyclase activation and the carboxyl-terminus of the IC(3) loop participates in both Galphas and Galpha16 coupling. The mutations studied also reduced both receptor phosphorylation and internalization in a manner that appeared directly linked to the alteration of Galphas and Galpha16 coupling.