Regulation and activities of phytosulfokine receptor PSKR1 in Arabidopsis thaliana
In Arabidopsis thaliana, the sulfated peptide growth factor phytosulfokine (PSK) is perceived by two plasma membrane-bound leucine-rich repeat receptor-like kinases. The PSK signal is mainly conveyed by PSKR1 and therefore this study focusses on the functional analysis of PSKR1. In particular, the activation mechanism, binding to calmodulin (CaM) and substrate phosphorylation were characterized. Mapping of PSKR1 autophosphorylation revealed that phosphosites are present in different regions of the cytoplasmic protein part including the juxtamembrane (JM) domain, all kinase subdomains and the C-terminus (CT). A functional analysis by point mutation showed that phosphorylation of amino acids in the activation segment is essential for PSKR1 activity. Amino acids, whose mutations strongly impacted kinase activity, are highly conserved in PSKR1 orthologues from higher plants. A homology model of the cytoplasmic part of PSKR1 shows a typical bilobal kinase structure and revealed that phosphorylated amino acids are positioned in catalytically essential structural elements. Phosphorylation of the JM domain impacted substrate phosphorylation in vitro and led in planta to PSK response in the root but not in the shoot. Binding of CaM to PSKR1 is dependent on calcium and reduced by autophosphorylation of PSKR1. The role of Ca2+ /CaM binding is unclear, but is not regulation of PSKR1 kinase activity. A functional PSKR1 kinase is essential to promote cell expansion. PSKR1 binds directly to the proton pumps Arabidopsis H+ -ATPases 1 and 2 (AHA1 and 2) in the plasma membrane. The autoinhibitory CT of AHA2 is phosphorylated by PSKR1 in vitro at activating positions. In planta, PSK induces an enhanced proton efflux in roots of Arabidopsis seedlings, whereas in the PSK-insensitive double receptor mutant no acidification takes place. This leads to the conclusion that AHAs are a substrate of PSKRs.