Mutation of the phosphorylatable residue Thr429 in Glu of the human VPAC1 led to a constitutively desensitized receptor.

The hVPAC1 receptor is rapidly phosphorylated and internalized by agonists but not re-expressed at the membrane after washing. Mutation of Ser/Thr residues in the C-terminus reduced phosphorylation but not internalization that was abolished only when all the phosphorylatable residues were mutated. Substitution of Thr429 by Glu mimicking a phosphothreonin led to a mutant with unchanged binding properties, decreased coupling to adenylate cyclase consisting in a reduced VIP potency, increased basal and VIP stimulated phosphorylation, preserved internalization followed by a rapid receptor re-expression. These are the expected characteristics of a constitutively desensitized receptor, putting forward the role of Thr429 phosphorylation in that process.