Diacylglycerol Signaling

3 Phorbol Esters and Diacylglycerol: The PKC Activators
show strong dependence on phosphatidylserine, whereas PKCe and PKCg do not
(Medkova and Cho 1998; Ananthanarayanan et al. 2003; Stahelin et al. 2004).
PKCh is uniquely responsive to cholesterol sulfate (Ikuta et al. 1994). Likewise,
different phospholipid compositions cause different structure activity relations
for different DAG receptors, shifting the relative potencies of ligands. Thus, higher
negative phospholipid compositions favored PDBu binding to PKCa, whereas a
low negative phospholipid composition favored the binding to RasGRP (Lorenzo
et al. 2000).
The concept of the hydrophobic switch makes the strong prediction that ligand
binding and receptor activation are coupled to the energy constraints of the overall
conformational change induced by the switching mechanism. All mechanisms that
hold the receptor in either an open or closed conformation will thus facilitate or
impede the binding and the coupled conformational change associated with the
hydrophobic switching. A powerful illustration of this principle is provided by
chimeras prepared between different PKC isoforms (Acs et al. 1997). Phorbol
ester induced translocation of the (C1 domain containing) regulatory domain of
PKCa with 30-fold greater potency when this domain was coupled to the catalytic
domain of PKCe in lieu of the normal PKCa catalytic domain. In addition, since the
different classes of DAG receptors presumably have differential conformational
consequences linked to this switching mechanism, it would thus be expected
that they would have different dependence on the different components of the
cellular environment.
3.10 Different Functional Roles for the C1 Domain
as a Hydrophobic Switch
The C1 domain functioning as a hydrophobic switch responsive to ligand binding
may have two distinct consequences. The more general consequence is that it
stabilizes association of the DAG receptor at hydrophobic surfaces, typically at
cellular membranes. In the case of the PKCs, this membrane association is also
associated with stabilization of an unfolded conformation of the enzyme, leading to
extraction of the pseudosubstrate region from the catalytic site and enzyme activation.
In the case of PKD, in contrast, enzymatic activation is a consequence of
phosphorylation, typically by PKC, and the role of the C1 domain is for membrane
association of the activated enzyme (Wang 2006). Since it is known that PKC
isoforms such as PKCd can be activated by tyrosine phosphorylation downstream
of oxidative stress (Kikkawa et al. 2002; Steinberg 2008), it is plausible in such
cases that C1 ligands under such conditions might again be influential for localization
of the enzyme but no longer for its enzymatic activity.
Nonetheless, absolute enzymatic activity is not the most relevant parameter for
determining biological consequences. For PKC or PKD, the ability to phosphorylate
downstream targets will depend not only on the intrinsic activity of the enzyme
but also on the concentration of its substrate, which in turn will be determined by
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