Biochemical and pathological research of acyl-CoA : lysophosphatidic acid acyltransferases.

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Lysophosphatidic acid (LPA) acyltransferase (LPAAT) is one of the key enzymes involved in de novo biosynthesis of glycerolipids, including phospholipids and triacylglycerol. Recent reports have suggested that LPAAT may be involved in signal transduction pathways and various diseases including the progression of cancer, an inflammation, and obesity through the synthesis of phosphatidic acid (PA), bioactive lipid second messenger. LPAATα is uniformly expressed throughout most tissues ; whereas LPAATβ is differentially expressed, with the highest level found in adipose tissues. Mutations in LPAATβ are reported to cause congenital generalized lipodystrophy, suggesting that β-isoform is involved in triacylglycerol synthesis and storage in adipocytes. To understand the physiological roles and properties of LPAAT isoforms, we investigated detailed enzymology of human LPAATα and β expressed in Sf9 and mammalian cells. Alignment of amino acid sequences from various acyltransferases (LPAATs, sn- … More glycerol-3-phosphate (G3P) acyltransferase) reveals four regions with strong homology (acyltransferase motifs). We examined the role of the highly conserved amino acid residues in these four motifs in human LPAATα activity by site-directed mutagenesis. We found that the H104, D109, F146, R149, E178, and R181, all play a role in LPAAT catalysis. R149 but not R181 could be exchangeable to basic amino acid K for the activity. T180 could be exchangeable to S. These results suggest that acyltransferase motifs seem to form a catalytically important site in this family of acyltransferases. In contrast, the roles of N- or C-terminal domains have not established. The program-based prediction and accessibility of trypsin to N- or C-terminal tag suggested that C-terminus of LPAATα or β isoforms faces to cytosolic or lumen side, respectively. In contrast, N-terminus of LPAATα and β was predicted that both faced to cytosolic side. N-terminal tag of LPAATα resisted tryptic digestion, however, addition of PA, the product of the enzyme, rendered them labile to trypsin. These results suggest that the N-terminus faces to cytoslic side but hided behind the membranes, and PA induced the conformational changes to expose of the N-terminus. Deletion of 25 amino acids at N-terminal domains of LPAATα (up to predicted first transmembrane domain) reduced the LPAAT activity, suggesting that these regions are not essential but affect for the activity. Deletion of 20 amino acids at C-terminal domains of LPAATα increased the activity, suggesting that these regions are not essential and intrinsically inhibited the activity. Taken together, these results suggested the regulatory roles of N-terminal and C-terminal domains of the enzyme. Less