On the nature of the plant ER exit sites

Abstract

In plants, the endoplasmic reticulum (ER) and Golgi bodies are not only in close proximity, but are also physically linked. This unique organization raises questions about the nature of the transport vectors carrying cargo between the two organelles. Same as in metazoan and yeast cells, it was suggested that cargo is transported from the ER to Golgi cisternae via COPII-coated vesicles produced at ribosome-free ER exit sites (ERES). Recent developments in mammalian cell research suggest, though, that COPII helps to select secretory cargo, but does not coat the carriers leaving the ER. Furthermore, it was shown that mammalian ERES expand into a tubular network containing secretory cargo, but no COPII components. Because of the close association of the ER and Golgi bodies in plant cells, it was previously proposed that ERES and the Golgi comprise a secretory unit that travels over or with a motile ER membrane. In this study, we aimed to explore the nature of ERES in plant cells and took advantage of high-resolution confocal microscopy and imaged ERES labelled with canonical markers (Sar1a, Sec16, Sec24). We found that ERES are dynamically connected to Golgi bodies and most likely represent pre-cis-Golgi cisternae. Furthermore, we showed fine tubular connections from the ER to Golgi compartments (ERGo tubules) as well as fine protrusions from ERES/Golgi cisternae connecting with the ER. We suggest that these tubules observed between the ER and Golgi as well as between the ER and ERES are involved in stabilizing the physical connection between ER and ERES/Golgi cisternae, but may also be involved in cargo transport from the ER to Golgi bodies.

Identification and characterization of the COPII vesicle‐forming GTPase Sar1 in Chlamydomonas

“…By expressing CrSar1‐YFP in the Chlamydomonas expression strain UVM11 (Barahimipour et al, 2016 ; Neupert et al, 2009 ), we observed a cytosolic accumulation pattern of the YFP signal, with punctate structures distributed in the cytosol (Figure 2a ). In Arabidopsis and tobacco, the Sar1‐containing punctate structures are closely associated with the ER, representing putative COPII assembly and ER exit sites (ERES) (McGinness et al, 2022 ; Takagi et al, 2020 ; Zeng et al, 2015 ). To further examine the CrSar1‐containing punctate structures in the alga, we next co‐expressed CrSar1‐YFP with the ER marker mCerulean‐HDEL (Rasala et al, 2014 ).…”

ER-to-Golgi traffickingviaa dynamic intermediatecis-Golgi tubular network in Arabidopsis

“…Why was this cis -Golgi network never observed before in plant cells? Actually, membrane tubules were suggested to exist between the ER and the plant Golgi for a long time 12,13,45 . However, previous immuno-EM experiments using cis -Golgi markers were all focused on the Golgi stack and, consistently with our results, all show a pearled membrane structure at the cis -side of the stack 7,40,46,47 .…”

“…In plant cells, due to the close proximity of the Golgi and ERES, it has been suggested that no equivalent of the ERGIC exists 6,12 . However, it has been described that some part of the ER, or some ER-derived protrusions, can from contacts with the Golgi apparatus 7,[13][14][15] . In addition, it has been observed that inducing Golgi movement by trapping the Golgi with optical tweezers results in the dynamic remodeling of the ER network, thereby suggesting that the ER and the Golgi are in physical contact 16 .…”

Keep in contact: multiple roles of endoplasmic reticulum–membrane contact sites and the organelle interaction network in plants

“…VAP27 is likely to participate in most of these connections according to the literature in mammalian systems and plants. (b–d) A few examples of ER–organelle interactions at the light microscopy level in plants, such as ER (GFP‐HDEL) and Golgi bodies (ST‐RFP; Bar, 1 μm; McGinness et al ., 2022); ER (GFP‐HDEL) and mitochondria (Mito‐mCherry; Bar, 2 μm; Li et al ., 2022); ER (RFP‐HDEL) and autophagosomes (NAP1‐GFP; Bar, 2 μm; Wang et al ., 2016a). All images were taken from tobacco leaf epidermal cells.…”