The
phytosulfokine receptor 1, PSKR1 is a membrane-localised leucine-rich repeat
receptor-like kinase that also possesses intrinsic guanylate cyclase (GC)
activity. The GC activity is conferred, in part, by the presence of a GC
catalytic centre that is embedded within its kinase domain. This unusual domain
architecture represents a novel class of GC-linked receptor kinases. This novel
class of kinases was unearthed using sequence homology-guided bioinformatic
data mining tools. Only four members of this new class of kinases have been
shown to possess both kinase and GC activity. Currently, there is a paucity of
information as to how this dual catalytic activity is regulated in these molecules;
therefore we set out to explore the regulatory factors that modulate the dual
catalysis in this unusual family of receptor kinases. Our functional studies on
PSKR1 demonstrate that calcium acts as a molecular switch regulating this dual
catalysis. Functional analysis of PSKR1 at different calcium concentrations
showed that calcium inhibits the kinase activity of PSKR1 in a concentration
dependant manner whilst on the other hand, enhancing the GC activity of PSKR1. Our
previous studies have also demonstrated that, cyclicGMP (a product of GC
activity) inhibits the kinase activity of PSKR1. Taken together, our
observations indicate that calcium and cyclicGMP act as molecular switches of
PSKR1-mediated signalling. Furthermore, our structural analysis of the
cytoplasmic domain of PSKR1 suggests that it exists as a reversible dimer in
solution. This observation may represent a physiological conformation of PSKR1.
In a separate experiment, using tandem mass spectrometry, we have mapped out
the phosphorylation pattern of the cytoplasmic domain of PSKR1. Our findings
show that the cytoplasmic domain of PSKR1 has 14 phosphorylation sites in its
cytoplasmic domain including 3 phospho-tyrosines. This current study presents a
mechanistic model of how calcium and phosphorylation act as bimodal switches
regulating the dual catalysis in PSKR1.