COPII Sec23 proteins form isoform-specific ER exit sites with differential effects on polarized growth

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Abstract
COPII, a coat of proteins that form vesicles on the ER, mediates vesicle traffic from the ER to the Golgi. In contrast to metazoans that have few genes encoding each COPII component, plants have expanded these gene families leading to the hypothesis that plant COPII has functionally diversified. Here, we analyzed the gene families encoding for the Sec23/24 heterodimer in the moss Physcomitrium (Physcomitrella) patens . In P. patens, Sec23 and Sec24 gene families are each comprised of seven genes. Silencing the Sec23/24 genes revealed isoform specific contributions to polarized growth, with the closely related Sec23D/E and Sec24C/D essential for protonemal development. Focusing on the Sec23 gene family, we discovered that loss of Sec23D alters ER morphology, increases ER stress, inhibits trafficking to the Golgi and to the plasma membrane in tip growing protonemata. In contrast, the remaining five Sec23 genes are dispensable for tip growth. While Sec23A/B/C/F/G do not quantitatively affect ER to Golgi trafficking in protonemata, they do contribute to secretion to the plasma membrane. Of the three highly expressed Sec23 isoforms in protonemata, Sec23G forms ER exit sites that are larger than Sec23B and Sec23D and do not overlap with Sec23D. Furthermore, ER exit sites labeled by Sec23B or Sec23G form in the absence of Sec23D. These data suggest that Sec23D/E form unique ER exit sites contributing to secretion that is essential for tip growing protonemata.

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