Identification of protein kinase C phosphorylation sites involved in desensitization of the histamine H1 receptor

: We studied whether direct phosphorylation plays a key role in protein kinase C-activating phorbol ester-mediated H1 receptor desensitization. Several potential protein kinase C-mediated phosphorylation sites were located in the third cytoplasmic loop form our cloning studies of H1 receptors. Ser396 and Ser398 were determined to be the phosphorylation sites by in vitro phosphorylation studies using synthetic peptides corresponding to the partial amino acid sequence of the third cytoplasmic loop. Mutant H1 receptors whose Ser396 or Ser398 were displaced by alanine were expressed in Chinese hamster ovary cells by site-directed mutagenesis. Characterization of these receptors revealed that Ser398, but not Ser396, was primarily responsible for protein kinase C-mediated H1 receptor desensitization.