Two basic amino acids in the second inner loop of the interleukin-8 receptor are essential for Galpha16 coupling

The involvement of basic residues of interleukin(IL)-8 receptors in coupling to the Gi and G16 proteins was investigated by using a series of IL-8 receptor mutants. Substitution of the basic amino acids in the third inner loop of the receptor does not alter the abilities of the receptor mutants to activate recombinant Galpha16 or phosphoinositide-specific phospholipase C (PLC) beta2 expressed in COS-7 cells. However, an IL-8 receptor mutant with double mutations at residues Lys158 and Arg159 of the second inner loop loses its abilities to activate Galpha16 but retains its ability to activate PLC beta2. The activation of PLC beta2 by an IL-8 receptor that is sensitive to pertussis toxin has been previously demonstrated to be mediated through Gbetagamma. Surprisingly, the IL-8 receptor mutants with substitution of Ala for either residue Lys158 or Arg159 can still activate Galpha16, which suggests that either of the two basic residues in the second inner loop of the IL-8 receptor is sufficient for Galpha16 coupling.