The Plant Science Education Unit, The Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0101, Japan.
BMC Plant Biol. 2010 Sep 14;10:200. doi: 10.1186/1471-2229-10-200.
Phosphoenolpyruvate carboxylase (PEPC) is a critical enzyme catalyzing the β-carboxylation of phosphoenolpyruvate (PEP) to oxaloacetate, a tricarboxylic acid (TCA) cycle intermediate. PEPC typically exists as a Class-1 PEPC homotetramer composed of plant-type PEPC (PTPC) polypeptides, and two of the subunits were reported to be monoubiquitinated in germinating castor oil seeds. By the large-scale purification of ubiquitin (Ub)-related proteins from lily anther, two types of PEPCs, bacterial-type PEPC (BTPC) and plant-type PEPC (PTPC), were identified in our study as candidate Ub-related proteins. Until now, there has been no information about the properties of the PEPCs expressed in male reproductive tissues of higher plants.
Expression analyses showed that lily BTPC (LlBTPC) and Arabidopsis BTPC (AtBTPC) were significantly expressed in pollen. The fusion protein AtBTPC-Venus localized in the cytoplasm of the vegetative cell (VC). Both LlBTPC and AtBTPC expression initiated after the last mitosis before pollen germination. Lily PTPC (LlPTPC) and monoubiquitinated LlPTPC (Ub-LlPTPC) remained at constant levels during pollen development. In late bicellular pollen of lily, LlBTPC forms a hetero-octameric Class-2 PEPC complex with LlPTPC to express PEPC activity.
Our results suggest that an LlBTPC:Ub-LlPTPC:LlPTPC complex is formed in the VC cytoplasm during late pollen development. Both LlBTPC and AtBTPC expression patterns are similar to the patterns of the appearance of storage organelles during pollen development in lily and Arabidopsis, respectively. Therefore, BTPC is thought to accelerate the metabolic flow for the synthesis of storage substances during pollen maturation. Our study provides the first characterization of BTPC in pollen, the male gametophyte of higher plants.
磷酸烯醇丙酮酸羧化酶（PEPC）是一种关键酶，可催化磷酸烯醇丙酮酸（PEP）的β-羧化作用，生成三羧酸（TCA）循环中间产物草酰乙酸。PEPC 通常以由植物型 PEPC（PTPC）多肽组成的 1 类 PEPC 同源四聚体形式存在，已有报道称，在萌发的蓖麻种子中，有两个亚基被单泛素化。通过百合花药中泛素（Ub）相关蛋白的大规模纯化，我们的研究鉴定了两种类型的 PEPC，即细菌型 PEPC（BTPC）和植物型 PEPC（PTPC），它们是候选的 Ub 相关蛋白。到目前为止，还没有关于高等植物雄性生殖组织中表达的 PEPC 特性的信息。
表达分析表明，百合 BTPC（LlBTPC）和拟南芥 BTPC（AtBTPC）在花粉中显著表达。融合蛋白 AtBTPC-Venus 定位于营养细胞（VC）的细胞质中。LlBTPC 和 AtBTPC 的表达都在花粉萌发前的最后一次有丝分裂后开始。百合 PTPC（LlPTPC）和单泛素化的 LlPTPC（Ub-LlPTPC）在花粉发育过程中保持不变。在百合的双核花粉后期，LlBTPC 与 LlPTPC 形成异八聚体 2 类 PEPC 复合物，表达 PEPC 活性。
我们的结果表明，在花粉晚期发育过程中，VC 细胞质中形成了 LlBTPC:Ub-LlPTPC:LlPTPC 复合物。LlBTPC 和 AtBTPC 的表达模式与百合和拟南芥花粉发育过程中储存器官出现的模式相似。因此，BTPC 被认为在花粉成熟过程中加速了储存物质的代谢流。我们的研究首次对高等植物雄性配子体花粉中的 BTPC 进行了表征。
