Characterization of bacterial-type phosphoenolpyruvate carboxylase expressed in male gametophyte 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.Phosphoenolpyruvate carboxylase (PEPC, EC4.1.1.31) catalyzes the irreversible β-carboxylation of phosphoenolpyruvate (PEP) to yield oxaloacetate and inorganic phosphate (Additional file 1). PEPC exists widely in plants, algae, and bacteria, but not in animals or fungi [1]. In plants, PEPC acts as an allosteric enzyme and is phosphorylated by PEPC protein kinase [1-3]. Active PEPC commonly consists of a plant-type PEPC (PTPC) homotetramer, and is typically inhibited by L-malate and aspartic acid and activated by glucose-6-phosphate (Glc-6-P). PEPC has been extensively studied in C4 and CAM photosynthesis, because it is a critical enzyme catalyzing the initial reaction of atmospheric CO2 fixation [1]. It also plays pivotal metabolic roles in nonphotosynthetic and C3 photosynthetic cells, particularly in the anaplerotic replenishment of the TCA cycle intermediates consumed during lipid synthesis [4], biosynthesis, and n