Phosphorylation of the phytosulfokine peptide receptor PSKR1 controls receptor activity

The phytosulfokine peptide receptor PSKR1 is modified by phosphorylation of its cytoplasmic kinase domain. We analyzed defined phosphorylation sites by site-directed mutagenesis with regard to kinase activity in vitro and receptor activity in planta. S696 and S698 in the juxtamembrane (JM) domain are phosphorylated in planta. The phosphomimetic S696D/S698D replacements resulted in reduced transphosphorylation activity of PSKR1 kinase in vitro but did not reduce autophosphorylation activity. Growth-promoting activity of the PSKR1(S696D/S698D) receptor isoform was impaired in the shoot but not in the root. The JM domain thus seems to be important for phosphorylation of a target protein required for shoot growth promotion. The phosphomimetic replacement T998D at the C-terminus (CT) abolished kinase activity in vitro but not receptor function in planta, indicating that additional levels of regulation exist in planta. A possible mode of receptor regulation is the interaction with regulatory proteins such as the calcium sensor calmodulin (CaM). We show that the previously reported binding of CaM2 to PSKR1 is calcium-dependent, occurs predominately to the hypophosphorylated soluble PSKR1 kinase, and does not significantly change PSKR1 kinase activity. In conclusion, our results show that peptide signaling of growth by PSKR1 is regulated by differential phosphorylation of the juxtamembrane and C-terminal domains of the intracellular receptor part and suggest that interaction of PSKR1 with CaM serves a function other than the regulation of kinase activity.