Diacylglycerol kinase that converts diacylglycerol/DAG into phosphatidic acid/phosphatidate/PA and regulates the respective levels of these two bioactive lipids (PubMed:9159104, PubMed:15544348, PubMed:18004883, PubMed:19744926, PubMed:22108654, PubMed:22627129, PubMed:23949095). Thereby, acts as a central switch between the signaling pathways activated by these second messengers with different cellular targets and opposite effects in numerous biological processes (PubMed:9159104, PubMed:15544348, PubMed:18004883, PubMed:19744926, PubMed:22108654, PubMed:22627129, PubMed:23949095). Also plays an important role in the biosynthesis of complex lipids (Probable). Does not exhibit an acyl chain-dependent substrate specificity among diacylglycerol species (PubMed:9159104, PubMed:19744926, PubMed:22108654). Can also phosphorylate 1-alkyl-2-acylglycerol in vitro but less efficiently and with a preference for alkylacylglycerols containing an arachidonoyl group (PubMed:15544348, PubMed:19744926, PubMed:22627129). The biological processes it is involved in include T cell activation since it negatively regulates T-cell receptor signaling which is in part mediated by diacylglycerol (By similarity). By generating phosphatidic acid, stimulates PIP5KIA activity which regulates actin polymerization (PubMed:15157668). Through the same mechanism could also positively regulate insulin-induced translocation of SLC2A4 to the cell membrane (By similarity). {ECO:0000250|UniProtKB:Q80UP3, ECO:0000269|PubMed:15157668, ECO:0000269|PubMed:15544348, ECO:0000269|PubMed:18004883, ECO:0000269|PubMed:19744926, ECO:0000269|PubMed:22108654, ECO:0000269|PubMed:22627129, ECO:0000269|PubMed:23949095, ECO:0000269|PubMed:9159104, ECO:0000305|PubMed:8626588}.; [Isoform 1]: Regulates RASGRP1 activity. {ECO:0000269|PubMed:11257115}.; [Isoform 2]: Does not regulate RASGRP1 activity. {ECO:0000269|PubMed:11257115}.