Abstract
Membrane proteins are synthesized and folded in the endoplasmic reticulum (ER), and continue their path to their site of residence along the secretory pathway. The COPII system has been identified as a key player for selecting and directing the fate of membrane and secretory cargo proteins. Selection of cargo proteins within the COPII vesicles is achieved by cargo receptors. The cornichon cargo receptor belongs to a conserved protein family found in eukaryotes that has been demonstrated to participate in the selection of integral membrane proteins as cargo for their correct targeting. Here it is demonstrated at the cellular level that rice cornichon OsCNIH1 interacts with OsHKT1;3 and, in yeast cells, enables the expression of the sodium transporter to the Golgi apparatus. Physical and functional HKT-cornichon interactions are confirmed by the mating-based split ubiquitin system, bimolecular fluorescence complementation, and Xenopus oocyte and yeast expression systems. The interaction between the two proteins occurs in the ER of plant cells and their co-expression in oocytes leads to the sequestration of the transporter in the ER. In the yeast cornichon mutant erv14, OsHKT1;3 is mistargeted, preventing the toxic effects of sodium transport in the cell observed in wild-type cells or in the erv14 mutant that co-expressed OsHKT1;3 with either OsCNIH1 or Erv14p. Identification and characterization of rice cornichon as a possible cargo receptor opens up the opportunity to improve our knowledge on membrane protein targeting in plant cells.
Highlights
Membrane transport proteins have to be selectively targeted to specific membranes to control ion and metabolite fluxes into and within the cell
After analysing the 19 interacting proteins, it was decided to focus on a protein homologous to Erv14p in yeast and cornichon in Drosophila and mammals, which has been shown to direct the trafficking of membrane proteins from the endoplasmic reticulum (ER) to the Golgi
The putative cargo receptor OsCNIH1 interacts with the Na+ transporter OsHKT1;3
Summary
Membrane transport proteins have to be selectively targeted to specific membranes to control ion and metabolite fluxes into and within the cell. Very little is known about the sorting mechanisms of plant transporters. Eukaryotic cells developed multiple systems to target proteins to their respective compartments. Once membrane proteins are synthesized and folded in the endoplasmic reticulum (ER), they are directed by specific protein–protein interactions through the secretory pathway to the site where they will dwell. Transport of membrane and secretory cargo proteins is mediated by COPII vesicles that help to select these cargoes from ER-resident proteins. Humans, and plants, there is evidence that selection of cargo proteins is mediated by the Sec subunit of the 2734 | Rosas-Santiago et al
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