Foreword

Transporters are membrane proteins widely expressed in various organisms, including bacteria, plants and mammals. In the field of life science including fundamental, technological, agricultural, medicine and pharmaceutical sciences, transporters have been investigated as a key functional protein to understand the membrane transport of nutrients and ethical drugs, etc., and their irreplaceable roles in life activities have been revealed by cumulative evidence. To promote further discoveries, Japan Transporter Research Association organized one special lecture, sixteen invited lectures and 80 poster presentations during its 12th annual meeting held in Sendai in July 2017. For the Current Topics section in this issue of the Biological and Pharmaceutical Bulletin, we have assembled four reviews and one regular article that cover the recent unique progress in transporter research ongoing in various academic disciplines.

The first review is “Transport of Amino Acids across the Vacuolar Membrane of Yeast: Its Mechanism and Physiological Role” by Kawano-Kawada et al. Under nutrient-rich conditions, considerable accumulation of amino acids is observed in the vacuoles of yeast cells, and the amino acid composition in the cytosol and in the vacuoles is known to differ markedly, suggesting the involvement of specific mechanisms in amino acid transport at the vacuolar membrane. Kawano-Kawada et al. review the studies with Saccharomyces cerevisiae, to describe transporters involved in the vacuolar compartmentalization of amino acids and their activity and physiological functions.

Namba and Murata contributed the second review, entitled “Organic Chemistry Research on the Mechanistic Elucidation of Iron Acquisition in Barley.” The study of iron acquisition in plants was carried out with 2′-deoxymugineic acid (DMA), a phytosiderophore of rice, for which efficient synthesis has been established. Namba and Murata review their organic chemistry approach to the mechanistic investigation of iron acquisition in graminaceous plants and describe the direct evidence of transporter-mediated internalization of mugineic acid into cells, illustrating the potential of DMA for use as a fertilizer to improve food production.

The third review is “Aberrant Uptake of a Fluorescent L-Glucose Analogue (fLG) into Tumor Cells Expressing Malignant Phenotypes” by Yamada. Glucose is one of the most fundamental nutrients and can have either a D- or an L-conformation. It is widely known that D-glucose is preferentially taken up by most cells to be used as an essential energy/carbon source. The stereoselective uptake of glucose was investigated, and Yamada describes research using fluorescent L-glucose analogues (fLGs) that are unique for identifying and characterizing living cancer cells, suggesting a promising new approach to the development of a method for clinical cancer detection.

“Zinc Transporters and the Progression of Breast Cancers” by Takatani-Nakase is the fourth review article in this series. Zinc (Zn), an essential heavy metal, works as a key factor in various biological events. Zinc-regulated transporters (Zrt) and iron-regulated transporters (Irt)-like protein (ZIP) and the Zn transporter (ZnT) families are involved in the membrane transport of Zn to contribute to Zn homeostasis and Zn signaling. Takatani-Nakase reviews the contributions of ZIP6, ZIP7 and ZIP10 to human breast cancer progression in terms of grade, size, stage, drug resistance, invasion, and metastasis and provides new insight into novel diagnostic and therapeutic strategies for breast cancer.

Finally, the regular article “Hypotaurine Is a Substrate of GABA Transporter Family Members GAT2/Slc6a13 and TAUT/Slc6a6” was contributed by Nishimura et al. Hypotaurine, a precursor of taurine, is a physiological antioxidant that circulates in fetal and adult plasma. The hypotaurine concentration is correlated with spermatozoa motility, and it was also reported that Ezrin gene (Vil2)-knockout mice showed fetal growth retardation and hypotaurine deficiency in fetal plasma. Nishimura et al. studied the carrier-mediated transport of hypotaurine, and their results suggest the major contribution of the taurine transporter (TAUT/Slc6a6) and γ-aminobutyric acid (GABA) transporter 2 (GAT2/Slc6a13) to the hypotaurine transport.

These articles clearly show the diversity of transporter research, and will lead to further discoveries in life sciences. The editors of this Current Topics thank all the authors for their contributions.