Available online at www.sciencedirect.com The intracellular transport of transporters: membrane trafficking of mineral transporters Kentaro Fuji 1 , Kyoko Miwa 1 and Toru Fujiwara 1,2 For mineral nutrients to be used by plants, they must be taken up from soil solutions into root cells and then transported to shoots. Mineral nutrient transporters play a central role in this process, and their expression and accumulation are known to be strictly regulated in response to change in nutrient conditions. Roots are cylindrically shaped organs with various types of cells. For the nutrients to move from soil solution toward the xylem they have to be transported across various types of cells. Nutrient condition-dependent accumulation and polar distributions of transporters in plant cells are established by membrane trafficking systems. The present article provides an overview of current findings regarding the membrane trafficking of mineral nutrient transporters and a discussion of future perspectives in this field of research. Addresses 1 Biotechnology Research Center, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan 2 Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan Corresponding author: Fujiwara, Toru (atorufu@mail.ecc.u-tokyo.ac.jp) Current Opinion in Plant Biology 2009, 12:699–704 This review comes from a themed issue on Cell Biology Edited by Jir ˘ı ´ Friml and Karin Schumacher Available online 14th October 2009 1369-5266/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2009.09.006 Introduction Plant cells contain a large number of transporter proteins for a wide range of ions and metabolites. To function properly and to maintain plant cell homeostasis, these proteins must be expressed in the appropriate cell types and in appropriate amount. The plant body, however, is not a simple collection of cells. For plants to function at the whole-body level, nutrients must be transported transcellulary, so that soil minerals taken up by the roots can be loaded into the xylem. The transporters are also important for nutrient distribution among the aerial por- tions of plants. Recent studies show that depending on environmental conditions, intracellular distributions of some transporters are strictly regulated and dramatically changed by post- translational regulatory mechanisms for nutrient homeo- stasis (Figure 1). It is obvious that these regulatory mechanisms include vesicle-mediated membrane traf- ficking. All membrane proteins, including mineral nutrient transporters, that are associated with the plasma mem- brane (PM) or endomembrane compartments are syn- thesized in the rough endoplasmic reticulum (ER) and then distributed to the different intracellular com- partments via secretory pathways. As discussed below, the intracellular localizations of nutrient transporters are robustly and constitutively maintained, yet change dynamically in response to a variety of mediators such as phytohormone and conditions such as nutrient levels. Exit from the ER Newly synthesized transporters in the ER are sorted to their appropriate subcellular locations via vesicle- mediated membrane trafficking but must first exit the ER before entering secretory pathway. At ER exit sites, these cargo proteins are selected based on direct recog- nition by COP-II coat protein complexes, which comprise several components such as Sar1, Sec23/Sec24, and Sec31/Sec13, and are sorted to the secretory pathway. The motifs required for exit from the ER have been identified in several transporters. Furthermore, it has been reported that the processing of some transporters is limited already at this early stage. KAT1 is an inward-rectifying potassium channel, which is localized to the PM. And it was shown that KAT1 has four intrinsic motifs for ER exit, categorized as the acidic DXE/DXD motifs, established in the ion chan- nels of mammalian cells [1]. Mutations in the first diacidic motif of KAT1 resulted in ER accumulation of the protein. As a result, the inward conductance was reduced [2  ]. It is demonstrated in vivo that at the ER exit site, KAT1 directly interacts with Sec24, a com- ponent of COP-II coat proteins, but the mutated KAT1 does not [3  ,4]. Gonzalez et al. reported that PHT1, a high-affinity phos- phate (Pi) transporter, must be targeted to the PM for correct functioning of the protein [5]. The targeting pathway for PHT1 has been shown to require PHF1 (phosphate transporter traffic facilitator1), a plant- specific SEC12-related protein localized on the ER. SEC12, a guanine nucleotide exchange factor, induces the activation of Sar1 GTPase and triggers the initiation www.sciencedirect.com Current Opinion in Plant Biology 2009, 12:699–704