br DGKs bind and regulate other signalling proteins Based
DGKs bind and regulate other signalling proteins Based on the evidence noted above, DGKs achieve specificity of function through a combination of post-translational modifications, the availability of cofactors, and through the availability and access to substrate DAG. DGKs appear to achieve an additional level of specificity by binding to protein partners in order to regulate their activity. This concept is consistent with an emerging body of evidence indicating that specificity in signal transduction is often achieved by gathering appropriate protein partners through scaffolding proteins . DGKs appear to associate with proteins that are regulated by either DAG or PA. DGKζ provides the best example of this type of regulation. It binds and regulates at least three proteins, each of which is significantly affected by either DAG or PA. For example, by binding to RasGRP1, a DAG-dependent Ras guanine nucleotide exchange factor, DGKζ regulated the active state of Ras . This regulation was selective: five other DGK isotypes did not significantly inhibit RasGRP1. Its inhibition of Ras was consistent with the phenotype of DGKζ knockout mice, which had hyperresponsive T cells, in part, due to prolonged Ras activation . Regulation of RasGRP proteins may be a common theme: we found that DGKζ associated with and regulated RasGRP3 (M.K.T and D.S.R., unpublished observations). DGKζ also associated with PKCα to regulate its activity . Their association was dynamic: once phosphorylated by the PKC, DGKζ no longer associated with it. Demonstrating the specificity of their interaction, we found that DGKζ did not bind or regulate PKCδ. In each case, the DGK modulated the activity of its protein partner by metabolizing DAG. Conversely, we found that DGKζ also associated with and regulated, by generating PA, human PIP5K type Iα . Based on the Poly (A) Tailing sale of the mammalian DGK family many more examples of regulation through specific interactions will likely emerge in the near future.
DGKε modulates signalling events through its specificity DAG kinases may also be responsible for enriching phosphatidylinositols with specific lipid components. Phosphatidylinositols are enriched at the sn−2 position with unsaturated fatty acids, usually arachidonate . While it may seem that the specific fatty acid components would not significantly affect the signalling ability of DAG, data suggest that some DAG targets, including PKCs, are specifically activated by unsaturated DAG . How the fatty acid components of DAG affect target proteins is unclear, but one could speculate that these fatty acids may help enrich DAG in membrane microdomains where other signalling components reside. In vitro, most DGKs do not distinguish between the fatty acid components of DAG, suggesting that in vivo, phosphatidylinositols maintain their unsaturated fatty acid enrichment by coupling PI-specific phospholipase C enzymes with DAG kinases and other enzymes involved in resynthesizing PIP2. Coupling these enzymes, would maintain the fatty acid components of PIP2. Indeed, we found that DGKζ co-immunoprecipitated with the phosphoinositide-specific PLCs β1 and γ (M.K.T. and B.L., unpublished observations) and with human PIPK type Iα . Tabellini et al.  demonstrated that DGKθ associated with PLCβ1 and human PIPK type Iα. DGKε is unique among the DAG kinases because it selectively phosphorylates DAG with an arachidonate—an unsaturated fatty acid—in the sn−2 position . This selectivity suggests that DGKε may have a more prominent role than other DGKs in enriching inositol phospholipids with unsaturated fatty acids. To examine the biological function of this DGK, we generated mice with targeted deletion of DGKε. Proper inositol lipid signalling is important for normal neuronal transmission, so in a collaborative effort, we studied seizure threshold in the mice. We found that DGKε null mice had significantly shorter seizures following electroconvulsive shock and they recovered faster than wild-type mice . Examination of brain lipids demonstrated reduced levels of arachidonate in both PIP2 and DAG in the DGKε-deficient mice. These experiments underscore the importance of maintaining proper lipid composition of phosphatidylinositols and DAG and indicate that DGKε fulfills this role in some neuronal tissues.