Phosphorylation-independent internalisation and desensitisation of the human sphingosine-1-phosphate receptor S1P3

Here we demonstrate that phosphorylation of the sphingosine-1-phosphate (S1P) receptor S1P 3 is increased specifically in response to S1P. Truncation of the receptor's carboxyl-terminal domain revealed that the presence of a serine-rich stretch of residues between Leu332 and Val352 was essential to observe this effect. Although agonist-occupied wild-type (WT) S1P 3 could be phosphorylated in vitro by G-protein-coupled receptor kinase 2 (GRK2), a role of S1P 3 phosphorylation in controlling S1P 3 –G q/11 coupling was excluded since A) a phosphorylation-resistant S1P 3 mutant desensitised in a manner indistinguishable from the WT receptor and was phosphorylated to a greater extent than the WT receptor by GRK2 in vitro, and B) co-expression with GRK2 or GRK3 failed to potentiate S1P 3 phosphorylation. S1P 3 phosphorylation was also not required for receptor sequestration away from the cell surface. Together, these data suggest that S1P 3 function is not subject to conventional regulation by GRK phosphorylation and that novel aspects of S1P 3 function distinct from classical G-protein coupling and receptor internalisation may be controlled its carboxyl-terminal domain.