br Introduction Acylglycerol acyltransferases AGATs are pred
Introduction Acylglycerol acyltransferases (AGATs) are predominantly responsible for triglyceride synthesis in the body, via two major pathways: the glycerol phosphate (GP) pathway (Kennedy, 1957) and the monoacylglycerol (MG) pathway (see Coleman and Lee, 2004, Yen et al., 2008). The final step of both pathways involves diacylglycerol and fatty acyl CoA being catalytically converted into triglyceride via two distinct DGAT (diacylglycerol O-acyltransferase, E.C.18.104.22.168) families, DGAT1 and DGAT2 (Cases et al., 1998, Oelkers et al., 1998). The two pathways differ in the mode of synthesis of diacylglycerols prior to catalysis by DGAT1 and/or DGAT2, with the GP pathway involving de novo synthesis from glycerol-3-phosphate and fatty acyl CoA, whereas the MG pathway uses partially hydrolyzed monoacylglycerols and fatty acyl CoA. This penultimate step in the MG pathway is catalyzed by Cyclopamine encoded by the monoacylglycerol acyltransferase (MGAT; 2-acylglycerol O-acyltransferase; EC 22.214.171.124) gene sub-family, for which three MGAT (also designated as MGOT and MOGAT) genes have been reported in humans (Yen et al., 2002, Yen and Farese, 2003, Cheng et al., 2003). The human DGAT2-like gene family comprises at least seven members, including DGAT2 (Cases et al., 2001), MGAT1 (Yen et al., 2002), MGAT2 (Yen and Farese, 2003), MGAT3 (Cheng et al., 2003), and three genes located together on the X chromosome (DGAT2L6, AWAT1 and AWAT2) (Turkish et al., 2005). The latter two genes encode enzymes with acyl CoA wax alcohol acyltransferase (AWAT) activity, and are specifically expressed in sebocytes (skin), and play a role in preventing surface desiccation by the formation of wax esters. The triglyceride forming pathways are differentially distributed in the body with the GP pathway being widely distributed and responsible for triglyceride synthesis in most tissues of the body. The MG pathway however is predominantly localized in specific cell types, including enterocytes (intestine), hepatocytes (liver) and adipocytes (adipose tissue), where large amounts of triglycerides are synthesized or stored. In particular, MG is the major pathway of the small intestine, where partially hydrolyzed fats are used to synthesize triglycerides following lipid ingestion (see reviews in Coleman and Lee, 2004, Yen et al., 2008). This study describes the predicted sequences, structures and phylogeny of diacylglycerol acyltransferase-like (DGAT) and monoacylglycerol acyltransferase-like (MGAT) genes and enzymes from eutherian (human and mouse) and marsupial (opossum) mammals and from a bony fish (zebrafish) species. Computational methods were used to predict the primary, secondary and transmembrane structures for these enzymes, as well as gene locations, exonic structures and sequences for MGAT- and DGAT-like genes, using published data from genome sequences. Predictions of MGAT- and DGAT-like enzyme sequences from a wider range of vertebrates were also used to examine the phylogeny and evolution of these genes and to identify conserved amino acid residues, including likely candidates for the active sites and substrate binding regions for these enzymes.
Materials and methods
Results and discussion
Conclusions BLAT analyses of the opossum and zebrafish genomes using the amino acid sequences reported for human and mouse DGAT1 and DGAT2-like protein subunits were undertaken to interrogate these genomes. Evidence is reported for an opossum DGAT1 gene and six DGAT2-like genes, including DGAT2, AWAT1, AWAT2, DGAT2L6, MGAT1, MGAT2 and MGAT3 genes. Evidence for two zebrafish and pufferfish DGAT1-like genes and four DGAT2-like genes (DGAT2, MGAT1, MGAT2 and MGAT3) is also described. Three of the opossum genes (AWAT1, AWAT2 and DGAT2L6) were closely localized on the X chromosome, which is comparable to the locations of these genes in human, mouse and several other mammalian genomes examined (see Supplementary Table). Two other opossum DGAT2-like genes (DGAT2 and MGAT2) were also closely located on chromosome 4, for which tandem locations for these genes was observed for other mammalian, amphibian and zebrafish genomes examined (see Supplementary Table 1). The predicted amino acid sequences, secondary and transmembrane structures for the opossum and zebrafish DGAT1 and DGAT2-like subunits showed a high degree of similarity with the corresponding human and mouse enzymes. Phylogenetic analyses undertaken with human, mouse, opossum, zebrafish and pufferfish DGAT1 and DGAT2-like proteins supported the designation of these enzymes with relevant families and sub-families previously described. It is likely that opossum and zebrafish enzymes perform similar functions to those reported for human and mouse in triacylglycerol (DGAT1, DGAT2 and DGAT2L6 subunits), wax (AWAT1 and AWAT2 for the opossum) and diacylglycerol (MGAT1, MGAT2 and MGAT3) biosynthesis.