Department of Biochemistry and Molecular Biology, Michigan State University, MI 48824, USA.
Plant J. 2010 Oct;64(2):267-79. doi: 10.1111/j.1365-313X.2010.04336.x. Epub 2010 Sep 16.
Cellular folates function as co-enzymes in one-carbon metabolism and are predominantly decorated with a polyglutamate tail that enhances co-enzyme affinity, subcellular compartmentation and stability. Polyglutamylation is catalysed by folylpolyglutamate synthetases (FPGSs) that are specified by three genes in Arabidopsis, FPGS1, 2 and 3, which reportedly encode plastidic, mitochondrial and cytosolic isoforms, respectively. A mutational approach was used to probe the functional importance of folate polyglutamylation in one-carbon metabolism and development. Biochemical analysis of single FPGS loss-of-function mutants established that folate polyglutamylation is essential for organellar and whole-plant folate homeostasis. However, polyglutamylated folates were still detectable, albeit at lower levels, in organelles isolated from the corresponding isozyme knockout lines, e.g. in plastids and mitochondria of the fpgs1 (plastidial) and fpgs2 (mitochondrial) mutants. This result is surprising given the purported single-compartment targeting of each FPGS isozyme. These results indicate redundancy in compartmentalised FPGS activity, which in turn explains the lack of anticipated phenotypic defects for the single FPGS mutants. In agreement with this hypothesis, fpgs1 fpgs2 double mutants were embryo-lethal, fpgs2 fpgs3 mutants exhibited seedling lethality, and fpgs1 fpgs3 mutants were dwarfed with reduced fertility. These phenotypic, metabolic and genetic observations are consistent with targeting of one or more FPGS isozymes to multiple organelles. These data confirm the importance of polyglutamylation in folate compartmentation, folate homeostasis and folate-dependent metabolic processes, including photorespiration, methionine and pantothenate biosynthesis.
细胞叶酸作为一碳代谢的辅酶，主要通过聚谷氨酸尾巴进行修饰，从而增强辅酶亲和力、亚细胞区室化和稳定性。聚谷氨酸化由叶酸多聚谷氨酸合成酶（FPGS）催化，在拟南芥中由三个基因（FPGS1、2 和 3）特异性编码，分别编码质体、线粒体和细胞质同工酶。本研究采用突变方法探讨了叶酸多聚谷氨酸化在一碳代谢和发育中的功能重要性。对单个 FPGS 功能丧失突变体的生化分析表明，叶酸多聚谷氨酸化对于细胞器和整株植物叶酸稳态是必需的。然而，在相应同工酶敲除系中分离的细胞器中仍然可以检测到聚谷氨酸化的叶酸，尽管水平较低，例如在 fpgs1（质体）和 fpgs2（线粒体）突变体的质体和线粒体中。鉴于每个 FPGS 同工酶都被认为具有单一的区室靶向性，这一结果令人惊讶。这些结果表明，区室化 FPGS 活性存在冗余性，这反过来又解释了单个 FPGS 突变体缺乏预期表型缺陷的原因。与这一假设一致，fpgs1 fpgs2 双突变体胚胎致死，fpgs2 fpgs3 突变体幼苗致死，fpgs1 fpgs3 突变体矮小，生育力降低。这些表型、代谢和遗传观察结果与一个或多个 FPGS 同工酶靶向多个细胞器相一致。这些数据证实了多聚谷氨酸化在叶酸区室化、叶酸稳态和叶酸依赖的代谢过程中的重要性，包括光呼吸、蛋氨酸和泛酸生物合成。
