High-curvature domains of the ER are important for the organization of ER exit sites in Saccharomyces cerevisiae

Abstract

SummaryProtein export from the endoplasmic reticulum (ER) to the Golgi apparatus occurs at specialized regions known as the ER exit sites (ERES). In Saccharomyces cerevisiae, ERES appear as numerous scattered puncta throughout the ER. We examined ERES within the peripheral ER, finding that the proteins comprising the ERES localize on high-curvature ER domains where curvature-stabilizing protein Rtn1 is present. Drtn1 Drtn2 Dyop1 cells have fewer high-curvature ER domains, but ERES accumulate at the remaining high-curvature ER domains on the edge of expanded ER sheets. We propose that membrane curvature is a key geometric feature for the regulation of ERES localization. We also investigated a spatial relationship between ERES and Golgi cisternae. Golgi cisternae in S. cerevisiae are unstacked, dispersed, and moving in the cytoplasm with cis-cisternae positioned adjacent to ERES, whereas trans-cisternae are not. Morphological changes in the ER of Drtn1 Drtn2 Dyop1 cells resulted in aberrant Golgi structures, including cis-and trans-markers, and there was reduced movement at ERES between expanded ER sheets and the plasma membrane.

Vesicles versus Tubes: Is Endoplasmic Reticulum-Golgi Transport in Plants Fundamentally Different from Other Eukaryotes?

“…This situation has also been reported for yeast, where ERESs were associated with high-curvature domains containing the membrane-curving ER protein reticulon1 (Okamoto et al, 2012), and we have preliminary evidence that in Arabidopsis, reticulons may interact with SEC12, the Sar1-guanine nucleotide exchange factor that recruits the GTPase to the ER membrane as part of the COPII coatbuilding process (Kriechbaumer and Hawes, unpublished data). Thus, we can conclude that plant ERESprobably requires a curved ER surface on which to form.…”

“…4). On the Golgi side, cis-cisternae show a significantly high probability of staying in the vicinity of the ERESs, whereas trans-cisternae do not (Okamoto et al, 2012).…”

Membrane curvature at a glance

“…In making vesicle trafficking intermediates, the initial bud formation might help to concentrate proteins that favor the bud curvature or, alternatively, help to exclude proteins that either do not have an adaptor to localize them to the bud or have an intrinsic curvature that forces them to leave the bud. The concentration of high-curvature-sensing and -inducing proteins, such as Sar1, the ESCRT complex or sorting nexin proteins, is also important for the organization of ER exit sites or endosomal sorting and recycling (Cullen and Korswagen, 2012;Okamoto et al, 2012).…”

Secretory Protein Biogenesis and Traffic in the Early Secretory Pathway

“…These small domains are marked by both the COPII coat proteins themselves and accessory proteins such as Sec16, and, in some cells, Sec12 (Rossanese et al 1999;Connerly et al 2005;Watson et al 2006). ERES are located throughout the ER, with a seemingly random distribution that may in fact correspond to regions of high local curvature induced by the ER membrane proteins, Rtn1, Rtn2, and Yop1 (Okamoto et al 2012). In related yeasts, these sites are dynamic, with the ability to form de novo, fuse, and divide (Bevis et al 2002).…”