Type 2 Diacylglycerol kinase Pleckstrin homology (PH) domain
DGK (also called DAGK) catalyzes the conversion of diacylglycerol (DAG) to phosphatidic acid (PA) utilizing ATP as a source of the phosphate. In non-stimulated cells, DGK activity is low and DAG is used for glycerophospholipid biosynthesis. Upon receptor activation of the phosphoinositide pathway, DGK activity increases which drives the conversion of DAG to PA. DGK acts as a switch by terminating the signalling of one lipid while simultaneously activating signalling by another. There are 9 mammalian DGK isoforms all with conserved catalytic domains and two cysteine rich domains. These are further classified into 5 groups according to the presence of additional functional domains and substrate specificity: Type 1 - DGK-alpha, DGK-beta, DGK-gamma - contain EF-hand motifs and a recoverin homology domain; Type 2 - DGK-delta, DGK-eta, and DGK-kappa- contain a pleckstrin homology domain, two cysteine-rich zinc finger-like structures, and a separated catalytic region; Type 3 - DGK-epsilon - has specificity for arachidonate-containing DAG; Type 4 - DGK-zeta, DGK-iota- contain a MARCKS homology domain, ankyrin repeats, a C-terminal nuclear localization signal, and a PDZ-binding motif; Type 5 - DGK-theta - contains a third cysteine-rich domain, a pleckstrin homology domain and a proline rich region. The type 2 DGKs are present as part of this Metazoan DGK hierarchy. They have a N-terminal PH domain, two cysteine rich domains, followed by bipartite catalytic domains, and a C-terminal SAM domain. Their catalytic domains and perhaps other DGK catalytic domains may function as two independent units in a coordinated fashion. They may also require other motifs for maximal activity because several DGK catalytic domains have very little DAG kinase activity when expressed as isolated subunits. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
Jiyao W et al.(2023). "The conserved domain database in 2023.",