Diacylglycerol kinase that converts diacylglycerol/DAG into phosphatidic acid/phosphatidate/PA and regulates the respective levels of these two bioactive lipids (PubMed:11719522). 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 (Probable). Has a higher activity with long-chain diacylglycerols like 1,2-di-(9Z-octadecenoyl)-sn-glycerol compared to 1,2-didecanoyl-sn-glycerol (By similarity). Specifically expressed in brain, it regulates neuron-specific morphological changes including neurite branching and neurite spine formation (By similarity). {ECO:0000250|UniProtKB:P49621, ECO:0000250|UniProtKB:Q6NS52, ECO:0000269|PubMed:11719522, ECO:0000305}. [Isoform 2]: Does not associate with membranes but has a diacylglycerol kinase activity. . This is the function of DGKB (diacylglycerol kinase beta, Ensembl gene identifier ENSG00000136267).