MAIGO5 Functions in Protein Export from Golgi-Associated Endoplasmic Reticulum Exit Sites inArabidopsis 

Abstract

Plant cells face unique challenges to efficiently export cargo from the endoplasmic reticulum (ER) to mobile Golgi stacks. Coat protein complex II (COPII) components, which include two heterodimers of Secretory23/24 (Sec23/24) and Sec13/31, facilitate selective cargo export from the ER; however, little is known about the mechanisms that regulate their recruitment to the ER membrane, especially in plants. Here, we report a protein transport mutant of Arabidopsis thaliana, named maigo5 (mag5), which abnormally accumulates precursor forms of storage proteins in seeds. mag5-1 has a deletion in the putative ortholog of the Saccharomyces cerevisiae and Homo sapiens Sec16, which encodes a critical component of ER exit sites (ERESs). mag mutants developed abnormal structures (MAG bodies) within the ER and exhibited compromised ER export. A functional MAG5/SEC16A-green fluorescent protein fusion localized at Golgi-associated cup-shaped ERESs and cycled on and off these sites at a slower rate than the COPII coat. MAG5/SEC16A interacted with SEC13 and SEC31; however, in the absence of MAG5/SEC16A, recruitment of the COPII coat to ERESs was accelerated. Our results identify a key component of ER export in plants by demonstrating that MAG5/SEC16A is required for protein export at ERESs that are associated with mobile Golgi stacks, where it regulates COPII coat turnover.

Unique COPII component AtSar1a/AtSec23a pair is required for the distinct function of protein ER export in Arabidopsis thaliana

“…1E). The observation on the association of AtSar1a punctate with the Golgi apparatus is consistent with previous studies using superresolution microscopy that showed that ERES are surrounded by the Golgi apparatus and that ERES shows more numbers than the Golgi (15,23). In contrast, most of the AtSar1c-GFP-labeled punctae were not Golgi-associated under superresolution microscopy.…”

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

“…With the exception of Sec13, which has also been found at the nuclear envelope (Yang et al, 2005), Sec23, Sec24, and Sec31 are predominantly localized at such areas. Three-dimensional projection reconstruction of confocal images followed by rendering analyses have shown that Sec16 is localized in cup-like structures where the ER assumes a high-degree curvature (Takagi et al, 2013;Fig. 2), supporting the intriguing possibility for specific requirements of ER membrane curvature for the ERES establishment and maintenance.…”

“…In live-cell imaging analyses, the Arabidopsis (Arabidopsis thaliana) COPII coat components (Sec13, Sec23, Sec24, and Sec31), when expressed in highly vacuolated leaf epidermal cells in tobacco (Nicotiana tabacum) and Arabidopsis, have been found in punctate structures that are associated with the ER and move with the Golgi stacks (Stefano et al, 2006;Hanton et al, 2007Hanton et al, , 2009Sieben et al, 2008;Wei and Wang, 2008;Faso et al, 2009;Takagi et al, 2013;Tanaka et al, 2013), which, in fully expanded plant cells, are highly dispersed and motile (Boevink et al, 1998;Stefano et al, 2014). The punctae labeled by the COPII coat proteins are commonly indicated as ER exit sites (ERESs).…”

The dynamics of plant plasma membrane proteins: PINs and beyond

“…Zelazny et al, 2009;Sorieul et al, 2011). SECRETION-ASSOCIATED AND RAS-RELATED 1 (SAR1), a Ras-like small GTP-binding protein, also appears to function in COPII vesicle assembly in plants, and plant orthologs of further constituents of COPII-coated vesicles, selective vesicle budding and cargo export from the ER have been identified (Bar-Peled and Raikhel, 1997;Osterrieder et al, 2010;Montesinos et al, 2012;Takagi et al, 2013). This is also true for elements of COPI-mediated retrograde transport, which mediates sorting back to the ER (Donohoe et al, 2007).…”