br Biological roles of DGK
Biological roles of DGKϵ
Summary and future perspectives The most NVP-CGM097 segment of DGKϵ, comprised of residues 20–42, appears to have no role in binding the lipid substrate DAG or in the acyl chain specificity of substrate phosphorylation. Nevertheless, the sequence in this segment of DGKϵ is highly conserved among multicellular eucaryotes, with few if any conservative substitutions. We show that this segment is important for anchoring the protein to the membrane. Although it has no function in in vitro assays of catalytic activity, it likely has an important function in the context of the whole cell. This function could include regulating the activity of the enzyme depending on whether this segment forms a transmembrane helix or a re-entrant helix. In silico calculations suggest the possibility of such a conformational change (Decaffmeyer et al., 2008). This conformational switch would not likely be observed with the current in vitro assays of catalytic activity that are conducted in mixed micelles. However, we are currently developing a liposome-based assay for DGKϵ. In addition to effects on catalysis, the hydrophobic segment may also have a role, in the context of the whole cell, in protein–protein interactions or in translocation of DGKϵ between membranes. These functions may be particularly important for DGKϵ since this enzyme catalyzes a step in the PI-cycle that has to segregate lipid substrates and products from other metabolic pathways. Membrane translocation is also important because steps in the PI-cycle take place both in the endoplasmic reticulum and in the plasma membrane. There is evidence for DGKϵ being present in both of these membranes. The catalytic mechanism of DGKϵ is conserved from the bacterial enzyme, DgkB. However, all mammalian forms of DGK are much larger than DgkB; including the smallest isoform, DGKϵ. This allows the mammalian DGKs to evolve additional specific properties. In the case of DGKϵ, one of these properties is the specificity for substrates containing arachidonic acid. This specificity is not a result of the presence of the hydrophobic segment near the amino terminus of DGKϵ. We suggest that a pattern of amino acid residues similar to that found to be important for the recognition of arachidonic acid by lipoxygenases is responsible for this specificity. This suggests that, as with lipoxygenases, DGKϵ binds the hydrophobic arachidonoyl group in a channel of the protein outside of the bilayer. The importance of the acyl chain specificity of DGKϵ is to allow this enzyme to contribute to the enrichment of lipid intermediates of the PI-cycle with 1-stearoyl-2-arachidonoyl species (Epand, 2015). Mutations or deletions of DGKϵ are associated with several disease processes.
Acknowledgements This work was supported by the Natural Sciences and Engineering Council of Canada, grant 9848 (to RME). We are also grateful to Christopher M. Yip and Amy Won for help with the AFM study and to Alba Guarne for helpful discussions regarding protein modeling.
Introduction Obesity is the result of a disturbance to energy homeostasis, which is maintained by the central nervous system (CNS). Hormone, nutrient, and satiety signals generated by peripheral metabolic tissues convey the body\'s energy status to key brain areas. This information is processed to produce the appropriate autonomic, endocrine, and behavioral outputs both for long-term (body weight maintenance) and short-term (meal initiation and satiation) energy balance. Consistent with its pivotal role, about 25% of all susceptibility genes in the last Human Obesity Gene Map and nearly all genes implicated in monogenic obesity are expressed in the brain . Furthermore, several genome-wide association studies in humans have also implicated single-nucleotide polymorphisms in several neuronal genes with predisposition to high BMI , , . Therefore, the CNS is central to energy balance and further insights into its role could be valuable in understanding the pathological mechanisms and genetic susceptibilities underlying obesity and related metabolic disorders.