Diacylglycerol Signaling

126 E. Rozengurt
Each translocation step is associated to a particular PKD domain and to rapid
and reversible interactions. The first step of PKD translocation is mediated by the
cys2 motif of the CRD, which binds to DAG produced at the inner leaflet of the
plasma membrane as a result of PLC stimulation (Rey et al. 2001a). Interestingly,
it has also been reported that cys2 and flanking sequences can directly bind to activated
Ga q (Oancea et al. 2003). In contrast, the cys1 recruits PKD to the Golgi
apparatus (Maeda et al. 2001). The second step, i.e., reversible translocation from
the plasma membrane to the cytosol, requires the phosphorylation of Ser 744 and
Ser 748 within the activation loop of PKD (Rey et al. 2001a) leading to its catalytic
activation (Rey et al. 2006). Active PKD is then imported, via its cys2 motif, into
the nucleus, where it transiently accumulates before being exported to the cytosol
through a CRM1-dependent nuclear export pathway that requires the PH domain of
PKD (Rey et al. 2001b).
Antigen-receptor engagement of B cells and mast cells induces rapid translocation
of PKD from the cytosol to the plasma membrane (Matthews et al. 2000a;
Matthews et al. 1999b). The plasma membrane translocation promoted by antigenreceptor
engagement is reversible and does not appear to involve the nuclear compartment
(Matthews et al. 2000a). These different observations emphasize the
notion that in addition to the structural determinants present in PKD, other factors,
including cell context, stimulus and scaffolding proteins also influence its intracellular
distribution. In this context, the A-kinase anchoring protein (AKAP-Lbc),
which possesses Rho-specific guanine nucleotide exchange activity and is linked to
Ga 12/13 signaling, forms a multiprotein complex that includes PKD, PKCh and PKA
that facilitates PKD translocation and activation (Carnegie et al. 2004). Previous
results also demonstrated that Ga 13 and activated Rho promote PKD activation
(Yuan et al. 2003; Yuan et al. 2001). These findings support the notion that GPCRs
utilize both G q and G 12/13 pathways to induce PKD translocation and activation in
their target cells. However, the elucidation of the precise contribution of different
G proteins to the early and late phases of PKD activation induced by GPCR agonists
requires further experimental work.
As in fibroblasts, PKD is cytosolic in unstimulated T cells, but it rapidly
polarizes to the immunological synapse in response to antigen/antigen presenting
cells (Spitaler et al. 2006). PKD translocation is determined by the accumulation
of DAG at the immunological synapse and changes in DAG accessibility of
the PKD-CRD. Unstimulated T cells have a uniform distribution of DAG at the
plasma membrane, whereas after T cell activation, a gradient of DAG is created
with a persistent focus of DAG at the center of the synapse. PKD is only transiently
associated with the immune synapse, indicating a fine tuning of PKD
responsiveness to DAG by additional regulatory mechanisms (Spitaler et al.
2006). These results reveal the immune synapse as a critical point for DAG and
PKD interaction during T cell activation.
PKD2 also undergoes reversible translocation from the cytosol to the plasma
membrane in response to GPCR stimulation (Rey et al. 2003a). The reversible
translocation of PKD2 requires PKC activity and, as in the case of PKD, it can
be prevented by inhibiting the translocation of PKCe (Rey et al. 2004). In gas-