The role of sec24 in protein export from the plant endoplasmic reticulum

Plant cells contain multiple mobile Golgi bodies. Golgi bodies receive cargo from specialized subdomains of the endoplasmic reticulum (ER), so-called ER export sites (ERES). How ERES operate in plant cells is largely uncharacterized. In mammals and yeast, the commonly recognized ER-to-Golgi transport model asserts that protein transport between these two organelles is mediated by vesicles. Formation of these vesicles is interceded by COPII and COPI coat complexes. COPII coat proteins assemble at ERES. The minimal components of the COPII coat comprise the following proteins: the GTPase Sar1, and two large heterodimeric complexes, Sec23/24 and Sec13/31. COPII vesicles are responsible for forward (anterograde) protein traffic from the ER to the Golgi apparatus. Proteins are constantly recycled from the Golgi back to the ER through a conserved backward (retrograde) pathway mediated by COPI coat proteins. Fusion of the anterograde and retrograde carriers with target membranes is mediated by a subset of specialized proteins called soluble N-ethyl maleimide sensitive factor attachment protein receptors (SNAREs). Studies conducted in mammalian and yeast systems also concluded that ER-to-Golgi SNARE proteins and membrane cargo proteins are concentrated into COPII vesicles through a direct interaction and binding with the pre-budding complex Sec23/24-Sar1. The COPII component distribution and their biological function in plant cells are largely uncharacterized. Therefore, through the study of the COPII protein Sec24, this work aimed (i) to investigate where and how protein transport between ER and Golgi occurs in plant cells, and (ii) to establish the importance of the anterograde and retrograde transport equilibrium in regulating the ER protein export. To do so, live cell imaging of a fluorescent protein fusion of Sec24 was used and the dynamics of this protein chimaera were followed in tobacco leaf epidermal cells. The imaging investigations were complemented by mutagenesis studies and biochemical analyses. The obtained results indicate that in plant cells Sec24 is localized at specific regions of the ER that represent mobile units continuously joined to the Golgi apparatus. From this study the importance of the balance between the anterograde and retrograde transport in protein ER export has also emerged. I have shown in fact, that blockage of the retrograde pathway using Arf1 mutants and COPI chemical inhibitor determines the collapse of the anterograde protein trafficking from the ER to the Golgi. Moreover, this study has shown that Sec24 is capable of an interaction with the SNAREs Sed5 and Sec22. This is a forward step in our understanding of the role of Sec24 in the mechanism of cargo selection and recruitment