Beschreibung:
<jats:p>
<jats:italic>PLCβ</jats:italic>
(
<jats:italic>Phospholipase Cβ</jats:italic>
) enzymes cleave phosphatidylinositol 4,5-bisphosphate (
<jats:italic>PIP2)</jats:italic>
producing
<jats:italic>IP3</jats:italic>
and
<jats:italic>DAG</jats:italic>
(diacylglycerol).
<jats:italic>PIP2</jats:italic>
modulates the function of many ion channels, while
<jats:italic>IP3</jats:italic>
and
<jats:italic>DAG</jats:italic>
regulate intracellular Ca
<jats:sup>2+</jats:sup>
levels and protein phosphorylation by protein kinase C, respectively.
<jats:italic>PLCβ</jats:italic>
enzymes are under the control of G protein coupled receptor signaling through direct interactions with G proteins
<jats:italic>Gβγ</jats:italic>
and
<jats:italic>
Gα
<jats:sub>q</jats:sub>
</jats:italic>
and have been shown to be coincidence detectors for dual stimulation of
<jats:italic>
Gα
<jats:sub>q</jats:sub>
</jats:italic>
and
<jats:italic>Gα</jats:italic>
<jats:sub>
<jats:italic>i</jats:italic>
</jats:sub>
-coupled receptors.
<jats:italic>PLCβs</jats:italic>
are aqueous-soluble cytoplasmic enzymes but partition onto the membrane surface to access their lipid substrate, complicating their functional and structural characterization. Using newly developed methods, we recently showed that
<jats:italic>Gβγ</jats:italic>
activates
<jats:italic>PLCβ3</jats:italic>
by recruiting it to the membrane. Using these same methods, here we show that
<jats:italic>
Gα
<jats:sub>q</jats:sub>
</jats:italic>
increases the catalytic rate constant,
<jats:italic>
k
<jats:sub>cat</jats:sub>
</jats:italic>
, of
<jats:italic>PLCβ3</jats:italic>
. Since stimulation of
<jats:italic>PLCβ3</jats:italic>
by
<jats:italic>
Gα
<jats:sub>q</jats:sub>
</jats:italic>
depends on an autoinhibitory element (the X-Y linker), we propose that
<jats:italic>
Gα
<jats:sub>q</jats:sub>
</jats:italic>
produces partial relief of the X-Y linker autoinhibition through an allosteric mechanism. We also determined membrane-bound structures of the
<jats:italic>
PLCβ3·Gα
<jats:sub>q</jats:sub>
</jats:italic>
and
<jats:italic>PLCβ3·Gβγ</jats:italic>
<jats:italic>(2)</jats:italic>
<jats:italic>
·Gα
<jats:sub>q</jats:sub>
</jats:italic>
complexes, which show that these G proteins can bind simultaneously and independently of each other to regulate
<jats:italic>PLCβ3</jats:italic>
activity. The structures rationalize a finding in the enzyme assay, that costimulation by both G proteins follows a product rule of each independent stimulus. We conclude that baseline activity of
<jats:italic>PLCβ3</jats:italic>
is strongly suppressed, but the effect of G proteins, especially acting together, provides a robust stimulus upon G protein stimulation.
</jats:p>