A cornichon protein controls polar localization of the PINA auxin transporter in Physcomitrium patens.

Newly synthesized membrane proteins pass through the secretory pathway, starting at the endoplasmic reticulum and packaged into COPII vesicles, to continue to the Golgi apparatus before reaching their membrane of residence. It is known that cargo receptor proteins form part of the COPII complex and play a role in the recruitment of cargo proteins for their subsequent transport through the secretory pathway. The role of cornichon proteins is conserved from yeast to vertebrates, but it is poorly characterized in plants. Here, we studied the role of the two cornichon homologs in the secretory pathway of the moss Physcomitrium patens. Mutant analyses revealed that cornichon genes regulate different growth processes during the moss life cycle by controlling auxin transport, with CNIH2 functioning as a specific cargo receptor for the auxin efflux carrier PINA, with the C terminus of the receptor regulating the interaction, trafficking and membrane localization of PINA.

The C-terminus of the cargo receptor Erv14 affects COPII vesicle formation and cargo delivery.

The endoplasmic reticulum (ER) is the start site of the secretory pathway, where newly synthesized secreted and membrane proteins are packaged into COPII vesicles through direct interaction with the COPII coat or aided by specific cargo receptors. Little is known about how post-translational modification events regulate packaging of cargo into COPII vesicles. The Saccharomyces cerevisiae protein Erv14, also known as cornichon, belongs to a conserved family of cargo receptors required for the selection and ER export of transmembrane proteins. In this work, we show the importance of a phosphorylation consensus site (S134) at the C-terminus of Erv14. Mimicking phosphorylation of S134 (S134D) prevents the incorporation of Erv14 into COPII vesicles, delays cell growth, exacerbates growth of sec mutants, modifies ER structure and affects localization of several plasma membrane transporters. In contrast, the dephosphorylated mimic (S134A) had less deleterious effects, but still modifies ER structure and slows cell growth. Our results suggest that a possible cycle of phosphorylation and dephosphorylation is important for the correct functioning of Erv14.

Plant and yeast cornichon possess a conserved acidic motif required for correct targeting of plasma membrane cargos.

The export of membrane proteins along the secretory pathway is initiated at the endoplasmic reticulum after proteins are folded and packaged inside this organelle by their recruiting into the coat complex COPII vesicles. It is proposed that cargo receptors are required for the correct transport of proteins to its target membrane, however, little is known about ER export signals for cargo receptors. Erv14/Cornichon belong to a well conserved protein family in Eukaryotes, and have been proposed to function as cargo receptors for many transmembrane proteins. Amino acid sequence alignment showed the presence of a conserved acidic motif in the C-terminal in homologues from plants and yeast. Here, we demonstrate that mutation of the C-terminal acidic motif from ScErv14 or OsCNIH1, did not alter the localization of these cargo receptors, however it modified the proper targeting of the plasma membrane transporters Nha1p, Pdr12p and Qdr2p. Our results suggest that mistargeting of these plasma membrane proteins is a consequence of a weaker interaction between the cargo receptor and cargo proteins caused by the mutation of the C-terminal acidic motif.

Identification of rice cornichon as a possible cargo receptor for the Golgi-localized sodium transporter OsHKT1;3.

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.