Department of Biotechnology, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
J Biol Chem. 2012 Apr 6;287(15):11934-41. doi: 10.1074/jbc.M111.330621. Epub 2012 Feb 15.
Phosphoserine phosphatase (PSP) catalyzes the dephosphorylation of phosphoserine to serine and inorganic phosphate. PSPs, which have been found in all three domains of life, belong to the haloacid dehalogenase-like hydrolase superfamily. However, certain organisms, particularly bacteria, lack a classical PSP gene, although they appear to possess a functional phosphoserine synthetic pathway. The apparent lack of a PSP ortholog in Hydrogenobacter thermophilus, an obligately chemolithoautotrophic and thermophilic bacterium, represented a missing link in serine anabolism because our previous study suggested that serine should be synthesized from phosphoserine. Here, we detected PSP activity in cell-free extracts of H. thermophilus and purified two proteins with PSP activity. Surprisingly, these proteins belonged to the histidine phosphatase superfamily and had been annotated as cofactor-dependent phosphoglycerate mutase (dPGM). However, because they possessed neither mutase activity nor the residues important for the activity, we defined these proteins as novel-type PSPs. Considering the strict substrate specificity toward l-phosphoserine, kinetic parameters, and PSP activity levels in cell-free extracts, these proteins were strongly suggested to function as PSPs in vivo. We also detected PSP activity from "dPGM-like" proteins of Thermus thermophilus and Arabidopsis thaliana, suggesting that PSP activity catalyzed by dPGM-like proteins may be distributed among a broad range of organisms. In fact, a number of bacterial genera, including Firmicutes and Cyanobacteria, were proposed to be strong candidates for possessing this novel type of PSP. These findings will help to identify the missing link in serine anabolism.
磷酸丝氨酸磷酸酶 (PSP) 催化磷酸丝氨酸去磷酸化生成丝氨酸和无机磷酸。PSP 存在于生命的三个领域中，属于卤代酸脱卤酶样水解酶超家族。然而，某些生物体，特别是细菌，缺乏经典的 PSP 基因，尽管它们似乎具有功能性的磷酸丝氨酸合成途径。在严格的化能自养和嗜热菌 Hydrogenobacter thermophilus 中，PSP 直系同源物的明显缺乏代表了丝氨酸生物合成中的一个缺失环节，因为我们之前的研究表明丝氨酸应该从磷酸丝氨酸合成。在这里，我们在 H. thermophilus 的无细胞提取物中检测到 PSP 活性，并纯化了两种具有 PSP 活性的蛋白质。令人惊讶的是，这些蛋白质属于组氨酸磷酸酶超家族，被注释为辅酶依赖性磷酸甘油酸变位酶 (dPGM)。然而，由于它们既没有变位酶活性，也没有活性所必需的残基，我们将这些蛋白质定义为新型 PSP。考虑到对 l-磷酸丝氨酸的严格底物特异性、动力学参数和无细胞提取物中的 PSP 活性水平，这些蛋白质在体内强烈暗示作为 PSP 发挥作用。我们还从 Thermus thermophilus 和拟南芥的“dPGM 样”蛋白中检测到 PSP 活性，表明 dPGM 样蛋白催化的 PSP 活性可能分布在广泛的生物体中。事实上，许多细菌属，包括厚壁菌门和蓝藻门，被认为是具有这种新型 PSP 的强有力候选者。这些发现将有助于确定丝氨酸生物合成中的缺失环节。
