Further experiments are underway to identify

the enzyme(s) responsible for TbLpn methylation. Figure 5 TbLpn is methylated in vivo . TbLpn was immunopurified from PF T. brucei cytosolic extracts using anti-TbLpn polyclonal antibodies as described under Material and Methods. As a negative control, the cytosolic extract was incubated in the absence of antibodies. Proteins present in the starting cytosolic fraction (C), as well as the bound (B) and unbound fractions (U) were separated on a 10% polyacrylamide gel and transferred to PVDF. The presence of TbLpn in the immune complexes was assessed by probing the membrane with anti-TbLpn polyclonal antibodies (1:1,000), followed by goat anti-rabbit IgGs. To determine whether TbLpn contains methylated arginines, the blot was probed with anti-mRG polyclonal antibodies (1:1,000) [52], followed by goat anti-rabbit IgGs. Signals were detected using chemiluminescence. P505-15 TbLpn displays phosphatidic acid phosphatase activity in vitro Lipin proteins are known to exhibit Mg2+-dependent phosphatidic acid phosphatase activity, catalyzing dephosphorylation of phosphatidic acid (PA) into diacylglycerol. The predicted amino acid sequence of TbLpn contains two conserved domains found in all lipins. In addition, two aspartic acid residues that have been shown to be essential

for enzymatic activity of yeast and mammalian lipins are also found in TbLpn. To determine whether recombinant TbLpn could catalyze dephosphorylation of phosphatidic acid, enzymatic assays were performed using the substrate 1,2-dioctanoyl-sn-glycero-3-phosphate MG-132 in vitro (DiC8 PA), Mg2+, and increasing amount of His-TbLpn. Following incubation at 30°C, the amount of Pi released was measured by reading the absorbance at 620 nm following O-methylated flavonoid the addition of PiBlue reagent. Figure

6 shows that recombinant TbLpn exhibits phosphatidate phosphatase activity, suggesting that TbLpn may play a role in the synthesis of phospholipids. From our data, we calculated that recombinant TbLpn has a specific activity of 200–225 nmol/min/mg. In contrast, the recombinant mutant in which the two conserved aspartic acid residues (Asp-445, Asp-447) were changed to alanines (His-DEAD) shows significantly less phosphatase activity. The calculated specific activity of 11–12 nmol/min/mg calculated for the mutant protein clearly implies that the two conserved aspartates are essential for this enzymatic activity. Figure 6 Recombinant TbLpn displays phosphatidic acid phosphatase activity. The enzymatic activity was measured by the release of phosphate from 1,2-dioctanoyl-sn-glycero-3-phosphate (DiC8 PA). The substrate was incubated with increasing amounts of either Liproxstatin-1 mouse His-TbLpn (black bars) or His-DEAD (white bars) recombinant proteins. The amount of phosphate released was measured using PiBlue reagent and recording the absorbance at 620 nm.